gpu: alpha-to-coverage support

include/SDL3/SDL_gpu.h

@@ -1767,7 +1767,7 @@ typedef struct SDL_GPUMultisampleState
     SDL_GPUSampleCount sample_count;  /**< The number of samples to be used in rasterization. */
     Uint32 sample_mask;               /**< Reserved for future use. Must be set to 0. */
     bool enable_mask;             /**< Reserved for future use. Must be set to false. */
-    Uint8 padding1;
+    bool enable_alpha_to_coverage;    /**< true enables the alpha-to-coverage feature. */
     Uint8 padding2;
     Uint8 padding3;
 } SDL_GPUMultisampleState;