Allow dual Y'CbCr/RGBA outputs.

The intended use case is to have Y'CbCr for encoding output but keep
RGBA around for easier preview. This causes a few effects to need to
send arrays around; it's a bit ugly to special-case them like this,
but I'm concerned about going generic wrt. how good various shader
compilers are to optimize if we went full multi-model everywhere
(without having tested, though).

ABI break due to changed EffectChain size.