Prepare for better understanding of 10- and 12-bit Y'CbCr.

Seemingly there is trickiness in how to interpret the integer
values that is different from what you'll typically see in R'G'B'
(or just GPUs and TV standards differ on that point as well).
Add an explanatory comment, and add a data member to YCbCrFormat
to prepare for correct 10/12-bit level handlings. We'll stay 8-bit
only for now, though, to avoid an API break for existing clients
for no good reason (there's no 10-bit input, really).