chroma: chain: use chroma_probe

Remove the not up to date pi_allowed_chromas. Use directly the
vlc_chroma_probe.h API to get the list of chroma fallbacks.

Remove recursivity handling:

Before, the chain module could load an other chain module for
conversions requiring an extra step (OPAQUE -> I420 -> RV24 for
example).

Now that the conversion list is acquired from start, we can add each
converter filters in an iterative loop.