... rather than when determining when to end the frame.
The opportunity to do so might not come when forced to drain,
and it seems nicer anyway to do so at parse wrapup time.
mpeg4videoparse might not push buffers while parsing. If those buffers
contain the DISCONT flag, it gets lost and downstream won't get any
buffer with the flag.
Fix it by adding the DISCONT to the next pushed buffer.
This makes backwards playback work.
This reverts commit d3219a261a5a7b7340b63c80010009a73b032759.
vop_coded 0 means that the previous frame should be repeated,
so it should have valid duration after all.
https://bugzilla.gnome.org/show_bug.cgi?id=683169
