[pinterf]

RGBA and YUVA permutations are working already.
But this latter idea can be confusing, e.g. you can specify only one YUV clip, and the default source plane order is YUV, which is good enough I think.
There is a rule that you can specify less input clips, and if no more clips found in the list, but there are still planes to process, the last one will be automatically used further on.