picture_pool: add dynamic pools support for opaque pictures

A better solution than the hack done in !6420 (8a31e7c0)

avcodec internal is often allocating more hw pics than what we configure. Even our va modules do allocate more pictures (cf. d3dv11).

This will avoid a deadlock when avcodec use more frames than us.

NB: The pool private data could be allocated in picture_pool instead of using picture private data (but it requires an extra malloc per picture).