avviddec: Accumulate ghost frame metadata into real frame
avviddec has a GstVideoCodecFrame
garbage collection scheme that releases frames without output buffers, if they were decoded before the one we're about to output.
Alter this scheme so we release frames without buffers that have their PTS before our current frame. Alter the PTS and duration of our output frame to cover the released frame and copy over the captions as well.
For #4167.