For now, this affects taking screenshots only.
If the video mixer is actually needed, we want to go through the video
mixer for screenshots too - which is why this function simply always
used the video mixer, and then copied the surface to RAM as RGBA.
Add reading the surface as nv12 if possible. If the format is correct,
and no special video processing (like deinterlacing) is used, then it's
read directly without going through the video mixer.
There's no particular reason for doing this, other than avoiding the
video mixer (like vo_opengl can do it now). Also, the next commit will
make use of this in vf_vdpaurb.c.
b40d8fdfa3