Changed SDL_GPU_TEXTUREFORMAT_INVALID to 0

This is more in line with SDL's convention and makes it easier to do zero-based structure initialization.

include/SDL3/SDL_gpu.h

@@ -384,7 +384,7 @@ typedef enum SDL_GPUIndexElementSize
  */
 typedef enum SDL_GPUTextureFormat
 {
-    SDL_GPU_TEXTUREFORMAT_INVALID = -1,
+    SDL_GPU_TEXTUREFORMAT_INVALID = 0,
 
     /* Unsigned Normalized Float Color Formats */
     SDL_GPU_TEXTUREFORMAT_A8_UNORM,