video_chroma: chain: allocate for mid-chain filters

Allocate a new picture directly for intermediate filters from the chain filter_chain, ie. except for the last one. Indeed, the callback can only allocate for the last filter from the chain, which output the same format as the chain filter_t itself.

Fix a crash when plugging a chain NV12->RV32 which creates first a converter (cvpx) NV12 -> I420 and then a I420 -> RV32 filter, leading to the cvpx filter allocating the wrong type of output frame.

Fixes #27721 (closed)