[FFmpeg-user] convert from bt601 to bt709

[Bob Maple]

> I think it's in an atom called ACLR;  I doubt ffmpeg writes this either...

So in looking at the source, ffmpeg _does_ write this Avid extension if
the codec is DNxHD, and hard-codes it as 'RGB range' which sounds wrong
but the Avid codecs give the right result and it looks right in
Quicktime Player.  This was converting v210 (Uncompressed 4:2:2 10-bit)
to DNxHD.

With that said, I just did some tests from a 10-bit RGB DPX source to
DNxHD and I can't make it right either.  The range gets re-scaled to
16-235 (even though it already is) and the color primaries aren't
right... but the math for this is starting to go out of my territory,
and trying to figure out the path through the code is going even further.