Add a shutdown D3D12 function to correctly free the D3D12 resources

[mamoniem]

When closing a D3D12 based application, there will be some errors due to the optick resources. Would be called like so:

void ShutdownD3D12()
{
#ifdef WITH_OPTICK
	OPTICK_GPU_SHUTDOWN_D3D12();
#endif

//the rest of d3d12 shutting down
}

this pull related to #121