cog: Improve RGB<->YCbCr matrixing

cog: Improve RGB<->YCbCr matrixing

YCbCr->RGB now has a 'bits' setting, which controls the number
of bits used for the matrix coefficients.  Choices are 6 or 8.
6 is low quality, but faster.
RGB->YCbCr now works.  Always works in 8 bit, since it's the same
speed as 6 bit.