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.
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