videorate: Fix max-duplication-time handling
Previously this would've only set discont=TRUE and then for all future buffers simply returned immediately.
Instead we also need to a) drain previous input until its buffer time b) update next_ts and base_ts accordingly for the gap c) actually store the new buffer after the gap so it can be used in the future and so the old buffer before the gap is gone
Also update the unit test accordingly so that it actually tests for this behaviour. Previously it only tested that after the gap we got no output at all.
CC @vivia, see also !131 (merged) which added this new property