avviddec: Fix how we get back the codec frame
avviddec: Fix how we get back the codec frame
With the new copy_opaque system, the corresponding frame is stored in the
picture opaque ref.
This also handles the case where the "regular" opaque might be empty in the
case of "DECODE_ONLY" frames, since it that field is set in `get_buffer2()`
which might not be called for those frames
See #3333 (closed)