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]
```

2 files changed, 7 insertions, 3 deletions

diff --git a/core/image.cpp b/core/image.cpp
index e90ed96499..e2b56c51dc 100644
--- a/core/image.cpp
+++ b/core/image.cpp
@@ -780,9 +780,9 @@ void Image::resize(int p_width, int p_height, Interpolation p_interpolation) {
 
 	// Setup mipmap-aware scaling
 	Image dst2;
-	int mip1;
-	int mip2;
-	float mip1_weight;
+	int mip1 = 0;
+	int mip2 = 0;
+	float mip1_weight = 0;
 	if (mipmap_aware) {
 		float avg_scale = ((float)p_width / width + (float)p_height / height) * 0.5f;
 		if (avg_scale >= 1.0f) {
diff --git a/core/os/memory.cpp b/core/os/memory.cpp
index be48c927b6..d212efe3cf 100644
--- a/core/os/memory.cpp
+++ b/core/os/memory.cpp
@@ -91,11 +91,15 @@ void *Memory::alloc_static(size_t p_bytes, bool p_pad_align) {
 	if (prepad) {
 		// Clang 5 wrongly complains about 's' being unused,
 		// while it's used to modify 'mem'.
+#ifdef __clang__
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunused-variable"
+#endif // __clang__
 		uint64_t *s = (uint64_t *)mem;
 		*s = p_bytes;
+#ifdef __clang__
 #pragma clang diagnostic pop
+#endif // __clang__
 
 		uint8_t *s8 = (uint8_t *)mem;