In scearios where a Temporal Unit is written right after reading it using the same
CBS context (av1_metadata, av1_frame_merge, etc), the reference frame state used
by the writer must not be the state that's the result of the reader having already
parsed the current frame in question.

This fixes writing Switch frames, and frames using short ref signaling.
Signed-off-by: James Almer <jamrial@gmail.com>