Only the collocated_ref of the current frame (i.e. HEVCContext.ref)
is ever used*, so move it to HEVCContext directly after ref.

*: This goes so far that collocated_ref was not even synced across
threads in case of frame-threading.
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>