Fix various warnings: [-Waddress], [-Wpointer-arith], [-Wwrite-strings], [-Wreturn-local-addr] and more

Fixes the following GCC 5 warnings:
```
core/os/file_access.cpp:49:19: warning: the address of 'FileAccess::create_func' will always evaluate as 'true' [-Waddress]
servers/audio_server.cpp:192:70: warning: comparison with string literal results in unspecified behaviour [-Waddress]

drivers/gles2/rasterizer_storage_gles2.cpp:4095:90: warning: NULL used in arithmetic [-Wpointer-arith]

modules/gdnative/register_types.cpp:237:3: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
platform/android/export/export.cpp:207:1: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]

modules/gdscript/gdscript.h:150:67: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:119:56: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:123:56: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:127:50: warning: returning reference to temporary [-Wreturn-local-addr]
servers/physics_2d/collision_object_2d_sw.h:131:52: warning: returning reference to temporary [-Wreturn-local-addr]

editor/plugins/skeleton_editor_plugin.cpp:34:36: warning: extra tokens at end of #include directive
modules/bullet/bullet_types_converter.cpp:31:9: warning: #pragma once in main file

editor/import/editor_scene_importer_gltf.cpp:1996:51: warning: name lookup of 'i' changed
modules/visual_script/visual_script_property_selector.cpp:402:45: warning: name lookup of 'E' changed
scene/gui/tree.cpp:1268:25: warning: name lookup of 'i' changed
scene/resources/visual_shader.cpp:808:32: warning: name lookup of 'i' changed
```

1 files changed, 6 insertions, 6 deletions

diff --git a/editor/import/editor_scene_importer_gltf.cpp b/editor/import/editor_scene_importer_gltf.cpp
index 85383fd69d..a6b754de3b 100644
--- a/editor/import/editor_scene_importer_gltf.cpp
+++ b/editor/import/editor_scene_importer_gltf.cpp
@@ -1910,15 +1910,15 @@ void EditorSceneImporterGLTF::_import_animation(GLTFState &state, AnimationPlaye
 		NodePath node_path;
 
 		GLTFNode *node = state.nodes[E->key()];
-		for (int i = 0; i < node->godot_nodes.size(); i++) {
+		for (int n = 0; n < node->godot_nodes.size(); n++) {
 
 			if (node->joints.size()) {
-				Skeleton *sk = (Skeleton *)node->godot_nodes[i];
+				Skeleton *sk = (Skeleton *)node->godot_nodes[n];
 				String path = ap->get_parent()->get_path_to(sk);
-				String bone = sk->get_bone_name(node->joints[i].godot_bone_index);
+				String bone = sk->get_bone_name(node->joints[n].godot_bone_index);
 				node_path = path + ":" + bone;
 			} else {
-				node_path = ap->get_parent()->get_path_to(node->godot_nodes[i]);
+				node_path = ap->get_parent()->get_path_to(node->godot_nodes[n]);
 			}
 
 			for (int i = 0; i < track.rotation_track.times.size(); i++) {
@@ -1993,8 +1993,8 @@ void EditorSceneImporterGLTF::_import_animation(GLTFState &state, AnimationPlaye
 						xform.basis.set_quat_scale(rot, scale);
 						xform.origin = pos;
 
-						Skeleton *skeleton = skeletons[node->joints[i].skin];
-						int bone = node->joints[i].godot_bone_index;
+						Skeleton *skeleton = skeletons[node->joints[n].skin];
+						int bone = node->joints[n].godot_bone_index;
 						xform = skeleton->get_bone_rest(bone).affine_inverse() * xform;
 
 						rot = xform.basis.get_rotation_quat();