Add support for Y'CbCr output split between multiple textures.

This is useful primarily for avoiding copies in later stages;
e.g., when rendering directly into a video encoder buffer.
We support both full planar and NV12-style interleaved Cb+Cr.
You still have to subsample chroma yourself, though; we don't
really support chains that diverge except in the final output node
(and changing resolution would definitely need a bounce;
and even worse, one in a non-fp16 intermediate format).