public override void Shutdown()
{
base.Shutdown();
_gpuProgramManager.Dispose();
_gpuProgramManager = null;
hardwareBufferManager.Dispose();
hardwareBufferManager = null;
//_rttManager.Dispose();
_rttManager = null;
_glSupport.Stop();
textureManager.Dispose();
textureManager = null;
_glInitialized = false;
}