message: Fix new compile errors in Metal EndFrameCapture
  Enabling -Wshorten-64-to-32 PR overlapped with this code landing
diff: @@ -184,8 +184,8 @@ bool WrappedMTLDevice::EndFrameCapture(DeviceOwnedWindow devWnd)
  MTL::CommandBuffer *mtlCommandBuffer = m_mtlCommandQueue->commandBuffer();
  MTL::BlitCommandEncoder *mtlBlitEncoder = mtlCommandBuffer->blitCommandEncoder();
  - NS::UInteger sourceWidth = mtlBackBuffer->width();
  - NS::UInteger sourceHeight = mtlBackBuffer->height();
  + uint32_t sourceWidth = (uint32_t)mtlBackBuffer->width();
  + uint32_t sourceHeight = (uint32_t)mtlBackBuffer->height();
  MTL::Origin sourceOrigin(0, 0, 0);
  MTL::Size sourceSize(sourceWidth, sourceHeight, 1);
  