Set a default shader entry point

The default should be the entrypoint generated by SDL_shadercross. That way it doesn't need to be hand-specified in the common workflow.

src/gpu/d3d12/SDL_gpu_d3d12.c

@@ -8056,7 +8056,6 @@ static void D3D12_INTERNAL_InitBlitResources(
     shaderCreateInfo.code_size = sizeof(D3D12_FullscreenVert);
     shaderCreateInfo.stage = SDL_GPU_SHADERSTAGE_VERTEX;
     shaderCreateInfo.format = SDL_GPU_SHADERFORMAT_DXBC;
-    shaderCreateInfo.entrypoint = "main";
 
     renderer->blitVertexShader = D3D12_CreateShader(
         (SDL_GPURenderer *)renderer,