mmal/codec: Explicitly count buffers in transit

mmal/codec: Explicitly count buffers in transit

The previous approach of calculating the buffers in transit was broken because
it assumed that buffers which were currently processed in the video output
stage are in transit. In fact only those buffers which are currently handed
off to the mmal output port of the video_decode plugin are to be counted as
buffers in transit. Fixing this avoids some framedrops due to missing/late
buffers.

Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>