Fix GCC 8 warnings about potentially unitialized variables

Fixes the following GCC 8 warnings:
```
core/image.cpp:730:44: warning: 'mip1_weight' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:293:20: warning: 'mip2' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:293:20: warning: 'mip1' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/audio_stream_preview.cpp:58:19: warning: 'vmax' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/audio_stream_preview.cpp:85:19: warning: 'vmin' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/editor_themes.cpp:306:53: warning: 'preset_contrast' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/plugins/animation_blend_space_2d_editor.cpp:459:27: warning: 'prev_idx' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/plugins/animation_blend_space_2d_editor.cpp:443:27: warning: 'prev_idx' may be used uninitialized in this function [-Wmaybe-uninitialized]
main/tests/test_oa_hash_map.cpp:57:29: warning: 'value' may be used uninitialized in this function [-Wmaybe-uninitialized]
modules/csg/csg.cpp:764:40: warning: 'max_angle' may be used uninitialized in this function [-Wmaybe-uninitialized]
modules/csg/csg_shape.cpp:1945:3: warning: 'face_count' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/3d/voxel_light_baker.cpp:1593:8: warning: 'cone_aperture' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/3d/voxel_light_baker.cpp:1592:6: warning: 'cone_dir_count' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/animation/animation_blend_space_2d.cpp:471:8: warning: 'mind' may be used uninitialized in this function [-Wmaybe-uninitialized]

core/os/memory.cpp:94: warning: ignoring #pragma clang diagnostic [-Wunknown-pragmas]
core/os/memory.cpp:95: warning: ignoring #pragma clang diagnostic [-Wunknown-pragmas]
core/os/memory.cpp:98: warning: ignoring #pragma clang diagnostic [-Wunknown-pragmas]
```

3 files changed, 5 insertions, 5 deletions

diff --git a/editor/audio_stream_preview.cpp b/editor/audio_stream_preview.cpp
index c5759ac076..5a94d41aba 100644
--- a/editor/audio_stream_preview.cpp
+++ b/editor/audio_stream_preview.cpp
@@ -50,7 +50,7 @@ float AudioStreamPreview::get_max(float p_time, float p_time_next) const {
 		time_to = time_from + 1;
 	}
 
-	uint8_t vmax;
+	uint8_t vmax = 0;
 
 	for (int i = time_from; i < time_to; i++) {
 
@@ -77,7 +77,7 @@ float AudioStreamPreview::get_min(float p_time, float p_time_next) const {
 		time_to = time_from + 1;
 	}
 
-	uint8_t vmin;
+	uint8_t vmin = 0;
 
 	for (int i = time_from; i < time_to; i++) {
 
diff --git a/editor/editor_themes.cpp b/editor/editor_themes.cpp
index 3cdbc4f188..c8292f1cb5 100644
--- a/editor/editor_themes.cpp
+++ b/editor/editor_themes.cpp
@@ -251,7 +251,7 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 
 	Color preset_accent_color;
 	Color preset_base_color;
-	float preset_contrast;
+	float preset_contrast = 0;
 
 	// Please, use alphabet order if you've added new theme here(After "Default" and "Custom")
 
diff --git a/editor/plugins/animation_blend_space_2d_editor.cpp b/editor/plugins/animation_blend_space_2d_editor.cpp
index c4f8cdc3d7..394b888d0e 100644
--- a/editor/plugins/animation_blend_space_2d_editor.cpp
+++ b/editor/plugins/animation_blend_space_2d_editor.cpp
@@ -434,7 +434,7 @@ void AnimationNodeBlendSpace2DEditor::_blend_space_draw() {
 
 		if (blend_space->get_snap().x > 0) {
 
-			int prev_idx;
+			int prev_idx = 0;
 			for (int i = 0; i < s.x; i++) {
 
 				float v = blend_space->get_min_space().x + i * (blend_space->get_max_space().x - blend_space->get_min_space().x) / s.x;
@@ -450,7 +450,7 @@ void AnimationNodeBlendSpace2DEditor::_blend_space_draw() {
 
 		if (blend_space->get_snap().y > 0) {
 
-			int prev_idx;
+			int prev_idx = 0;
 			for (int i = 0; i < s.y; i++) {
 
 				float v = blend_space->get_max_space().y - i * (blend_space->get_max_space().y - blend_space->get_min_space().y) / s.y;