[rtextures] Uninitialized thread-locals in stbi

[jbarthelmes]

Issue description

• I had an issue where textures were loaded upside-down.
• As it turns out, stb_image has thread-local variables to make an image be flipped after loading here.
• With -O3, these variables are not initialized and contain garbage. With -O0 they are zeroed apparently.
• As a result, they evaluate to true and the STBI image loading code took the branch (if (stbi__vertically_flip_on_load)) to vertically flip the image.

Environment

clang 16.0.6 (from zig 0.12.0-dev.252+5dc2db805), mingw64, -O3 (-Doptimize=ReleaseFast)

Code Example

examples/textures/textures_image_loading.c

Suggested Fix

1. Either the thread-locals are kept and initialized with stbi_set_flip_vertically_on_load_thread before being used, or
2. I prefer disabling them with #define STBI_NO_THREAD_LOCALS in rtextures.c (1-line PR will follow)

[raysan5]

@jbarthelmes good catch! thanks for the review!