Fix warnings for comparison between signed and unsigned integers [-Wsign-compare]

Also turn off -Wsign-compare warnings in the future, we do not consider them important.

Fixes the following GCC 5 warnings:
```
core/node_path.cpp:279:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
core/oa_hash_map.h:169:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
core/oa_hash_map.h:314:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/gles2/shader_gles2.cpp:985:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/gles3/rasterizer_storage_gles3.cpp:1075:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/pulseaudio/audio_driver_pulseaudio.cpp:343:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/editor_plugin.cpp:525:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/editor_properties_array_dict.cpp:747:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/plugins/spatial_editor_plugin.cpp:2078:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/plugins/spatial_editor_plugin.cpp:4096:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
editor/plugins/sprite_editor_plugin.cpp:100:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/cvtt/image_compress_cvtt.cpp:122:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/cvtt/image_compress_cvtt.cpp:134:77: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/cvtt/image_compress_cvtt.cpp:339:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/etc/image_etc.cpp:222:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/gdnative/register_types.cpp:242:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/gdnative/register_types.cpp:258:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/opensimplex/simplex_noise.cpp:200:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/opensimplex/simplex_noise.cpp:222:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
modules/opensimplex/simplex_noise.cpp:246:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/android/export/export.cpp:1085:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/android/export/export.cpp:1489:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/android/export/export.cpp:1623:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/iphone/export/export.cpp:206:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/iphone/export/export.cpp:356:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/iphone/export/export.cpp:406:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
platform/iphone/export/export.cpp:493:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/audio_stream_player_3d.cpp:420:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/resources/audio_stream_sample.cpp:565:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/resources/audio_stream_sample.cpp:571:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio/audio_rb_resampler.cpp:156:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
```

The following warnings were not fixed, as they implied casting for no gain:
```
core/io/packet_peer.cpp:228:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
core/io/resource_format_binary.cpp:109:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/gles2/rasterizer_scene_gles2.cpp:144:57: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/unix/file_access_unix.cpp:249:46: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/voxel_light_baker.cpp:889:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/voxel_light_baker.cpp:1020:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/voxel_light_baker.cpp:1154:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/3d/voxel_light_baker.cpp:2255:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
scene/resources/bit_mask.cpp:336:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio/audio_stream.cpp:141:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio/audio_stream.cpp:150:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio/audio_stream.cpp:154:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio_server.cpp:86:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
servers/audio_server.cpp:89:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
```

1 files changed, 4 insertions, 4 deletions

diff --git a/platform/iphone/export/export.cpp b/platform/iphone/export/export.cpp
index aae9d97a28..63bc4a519b 100644
--- a/platform/iphone/export/export.cpp
+++ b/platform/iphone/export/export.cpp
@@ -203,7 +203,7 @@ void EditorExportPlatformIOS::get_export_options(List<ExportOption> *r_options)
 	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "optional_icons/spotlight_40x40", PROPERTY_HINT_FILE, "*.png"), "")); // Spotlight
 	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "optional_icons/spotlight_80x80", PROPERTY_HINT_FILE, "*.png"), "")); // Spotlight on devices with retina display
 
-	for (int i = 0; i < sizeof(loading_screen_infos) / sizeof(loading_screen_infos[0]); ++i) {
+	for (unsigned int i = 0; i < sizeof(loading_screen_infos) / sizeof(loading_screen_infos[0]); ++i) {
 		r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, loading_screen_infos[i].preset_key, PROPERTY_HINT_FILE, "*.png"), ""));
 	}
 
@@ -353,7 +353,7 @@ Error EditorExportPlatformIOS::_export_icons(const Ref<EditorExportPreset> &p_pr
 	DirAccess *da = DirAccess::open(p_iconset_dir);
 	ERR_FAIL_COND_V(!da, ERR_CANT_OPEN);
 
-	for (int i = 0; i < (sizeof(icon_infos) / sizeof(icon_infos[0])); ++i) {
+	for (unsigned int i = 0; i < (sizeof(icon_infos) / sizeof(icon_infos[0])); ++i) {
 		IconInfo info = icon_infos[i];
 		String icon_path = p_preset->get(info.preset_key);
 		if (icon_path.length() == 0) {
@@ -403,7 +403,7 @@ Error EditorExportPlatformIOS::_export_loading_screens(const Ref<EditorExportPre
 	DirAccess *da = DirAccess::open(p_dest_dir);
 	ERR_FAIL_COND_V(!da, ERR_CANT_OPEN);
 
-	for (int i = 0; i < sizeof(loading_screen_infos) / sizeof(loading_screen_infos[0]); ++i) {
+	for (unsigned int i = 0; i < sizeof(loading_screen_infos) / sizeof(loading_screen_infos[0]); ++i) {
 		LoadingScreenInfo info = loading_screen_infos[i];
 		String loading_screen_file = p_preset->get(info.preset_key);
 		if (loading_screen_file.size() > 0) {
@@ -490,7 +490,7 @@ private:
 	static String _hex_pad(uint32_t num) {
 		Vector<char> ret;
 		ret.resize(sizeof(num) * 2);
-		for (int i = 0; i < sizeof(num) * 2; ++i) {
+		for (unsigned int i = 0; i < sizeof(num) * 2; ++i) {
 			uint8_t four_bits = (num >> (sizeof(num) * 8 - (i + 1) * 4)) & 0xF;
 			ret.write[i] = _hex_char(four_bits);
 		}