pthread_frame: change the criterium for updating thread contexts

pthread_frame: change the criterium for updating thread contexts

Currently the next thread's context is updated from the previous one's
if the codec descriptor is not marked as intra-only. That is not
entirely correct, since that property does not necessarily imply
anything about how a specific decoder implementation behaves.

Instead, use the presence of the update_thread_context() callback to
decide whether an update should be performed. Fixes races in CFHD,
should cause no behaviour change in any other decoders.