Semi-important, because --hwdec=dxva2 outputs NV12, and we really don't
want people to end up with the "old" StretchRect method.
Unfortunately, I couldn't actually get it to work. It seems most D3D
drivers (including the wine D3D implementation) reject D3DFMT_A8L8,
and I could not find any other 2-channel 8 bit Direct3D 9 format. It
seems newer D3D APIs have DXGI_FORMAT_R8G8_UNORM, but there's no way
to get it in D3D9.
Still pushing this; maybe it actually works on some drivers.
