d3d11window_win32: fix crash on RC unprepare() vs window_proc()
requested to merge aslobodeniuk/gstreamer:d3d11window_win32_fix_rc_on_message_when_stopping into main
Unprepare method posts WM_GST_D3D11_DESTROY_INTERNAL_WINDOW command to the window queue, and from that moment considers internal_hwnd to be released, and so it sets it to null. The problem is that it's possible that right at that moment the window thread might be already processing some other command, or just another command might be already in the queue. On practice we met a crash when WM_PAINT got processed in between (unprepare already finished and WM_GST_D3D11_DESTROY_INTERNAL_WINDOW was not handled yet)