This is in preparation for further commits that will stop
using ThreadFrame for frame-threaded codecs that don't use
ff_thread_(await|report)_progress(); the API for those codecs
having inter-frame depdendencies will live in threadframe.h.
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>