Alexandre Janniaux authored Jan 20, 2023 and François Cartegnie committed Jan 21, 2023

Allocate a new picture by forwarding the host filter whenever the last
filter of the cvpx chain asked a new video buffer.

The host filter and last CVPX buffer are supposed to have the same
format, but the host filter might play a different role in its parent
filter chain than the requesting filter. In particular, filter chain
will try to detect the last filter, which can only work for filters
belonging to the chain.

Fix a crash when plugging a chain NV12->RV32 which creates first a
converter (cvpx) CVPX_NV12 -> CVPX_I420 -> I420 and then another
converter I420 -> RV32 filter, leading to the cvpx filter allocating
the wrong type of output frame.

Fixes #27721