Reworked the way the vout select the pictures to be displayed.
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.