direct3d11: implement the pixel format fallback
direct3d11: implement the pixel format fallback

NV12 is not supported in Windows 7 [1] so we need to fallback to RGBA

1: https://msdn.microsoft.com/en-us/library/windows/desktop/bb173059%28v=vs.85%29.aspx#DXGI_FORMAT_NV12

Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

krichter722/vlc,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,xkfz007/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc