...Identical, except that Cb and Cr are swapped (and so I swapped the writes
to the display buffers elsewhere in the code). IYUV is an alias for I420;
unfortunately, for my mga driver, Xv only knows about "I420", but SDL only
knows about "IYUV", so hardware overlays never got used.
YV12 seems pretty universally supported by everyone.
o Fixed yuvplay to write to/display overlay properly.
