Implements unrotated planar YUV 4:2:0 -> RGB24 conversions in Y2R.
Currently only the Y (luma) channel is used, so the results don't
contain color. This will be added in a later PR at some point.

This is enough to get all currently know Moflex videos to decode. (Some
don't display on-screen due to seemingly unrelated reasons.)

Thanks to @archshift for doing the initial implementation which I
cleaned up and then fixed the 8x8 block mode.