avcodec/mmaldec: use decoupled dataflow
MMAL is an fundamentally an asynchronous decoder, which was a bad fit
for the legacy dataflow API. Often multiple packets are enqueued before
a flood of frames are returned from MMAL.
The previous lockstep dataflow meant that any delay in returning packets
from the VPU would cause ctx->queue_decoded_frames to grow with no way
of draining the queue.
Testing this with mpv streaming from a live RTSP source visibly reduced
latency introduced by frames waiting in queue_decoded_frames from
roughly 2s to 0.
Tested-by: Cameron Gutman <aicommander@gmail.com>
Signed-off-by: Ho Ming Shun <cyph1984@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>