Profiles 1256 & 1270 (currently) signal at the frame header and MB
levels the colorspace used, either RGB or YUV. While a MB-level
varying colorspace is not supported, whether it is constant can be
tracked so as to determine the exact colorspace.

This requires having bitdepth and the ACT and 4:4:4 flags, in turn
needing the CID. Because setting those before having validated
enough things may result in invalid/unset DSP fucntions, setting
the bitdepth in the context is delayed.

It is not tested against a true RGB sequence, though.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>