Use AV1DecContext's current_obu to access the original OBUs, and
feed them to videotoolbox, rather than the bare slice data passed
via decode_slice.
This requires a small addition to AV1DecContext, for keeping track
of the current range of OBUs that belong to the current frame.
Co-authored-by: Ruslan Chernenko <ractyfree@gmail.com>
Co-authored-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Martin Storsjö <martin@martin.st>
