It allows to:
 - properly support filters that do not have 1:1 input/output relation
 (yadif2x, bob, ... are back).
 - filter only once each picture (for the 'static' filter chain). It
 fixes filters with state behavior.
 - display as soon as possible the first frame after a flush.

There are still a few issues:
 - when paused, on filter changes, the picture displayed step forward the
 first time.
 - preventive frame dropping is not enough agressive.