This should halve the size of frames in memory for frames with 8-bit
color components, which is the majority of videos.
Calculation of the size of subsampled planes has also been consolidated
into a struct. There are likely some places that will still need to
change over to this, but it should prevent issues due to differing
handling of rounding/ceiling.
BT.2020 will mainly be used with bit depths greater than 8, so having
this specialization is mostly pointless until we use fixed-point math
for higher bit depths.
