Reland of 30d29618, except actually tested this time.
The VideoToolbox encoder gives I-frames and P-frames a priority of 1,
but the RTMP output code expects I-frames to have priority 3 and
P-frames to have priority 2.
30d29618 changed the priority of all frames that aren't I-frames, but
that included B-frames as well as P-frames. B-frames are given a
priority of 0 by VideoToolbox and changing that priority causes
artifacts for reasons I don't understand. So ignore the B-frames by
ignoring slice packets with priority 0.
5d3fbc15ae