Interactive/User filters are allowed to output more than one frame at a time.
(it could also be deinterlacers or fps filters added manually)

This introduces some timing differences in the processing time we use to adjust
the estimated display time. The drain may be faster or slower than the regular
filtering call.

Filter (chains) that only output one picture at the time won't be affected.

Before this patch, extra pictures from interactive filters were simply dropped
and never displayed.