GPU: fix SDL_GetGPUSwapchainTextureFormat error return value

src/gpu/d3d11/SDL_gpu_d3d11.c

@@ -5348,7 +5348,7 @@ static SDL_GPUTextureFormat D3D11_GetSwapchainTextureFormat(
 
     if (windowData == NULL) {
         SDL_LogError(SDL_LOG_CATEGORY_GPU, "Cannot get swapchain format, window has not been claimed!");
-        return 0;
+        return SDL_GPU_TEXTUREFORMAT_INVALID;
     }
 
     return windowData->textureContainer.header.info.format;

src/gpu/metal/SDL_gpu_metal.m

@@ -3563,7 +3563,7 @@ static SDL_GPUTextureFormat METAL_GetSwapchainTextureFormat(
 
     if (windowData == NULL) {
         SDL_LogError(SDL_LOG_CATEGORY_GPU, "Cannot get swapchain format, window has not been claimed!");
-        return 0;
+        return SDL_GPU_TEXTUREFORMAT_INVALID;
     }
 
     return windowData->textureContainer.header.info.format;

src/gpu/vulkan/SDL_gpu_vulkan.c

@@ -10132,12 +10132,12 @@ static SDL_GPUTextureFormat VULKAN_GetSwapchainTextureFormat(
 
     if (windowData == NULL) {
         SDL_LogError(SDL_LOG_CATEGORY_GPU, "Cannot get swapchain format, window has not been claimed!");
-        return 0;
+        return SDL_GPU_TEXTUREFORMAT_INVALID;
     }
 
     if (windowData->swapchainData == NULL) {
         SDL_LogError(SDL_LOG_CATEGORY_GPU, "Cannot get swapchain format, swapchain is currently invalid!");
-        return 0;
+        return SDL_GPU_TEXTUREFORMAT_INVALID;
     }
 
     return SwapchainCompositionToSDLFormat(