assrender: fix seeking backwards
Use proper API to flush libass events when we do a flushing seek, and also do it in FLUSH_STOP rather than FLUSH_START, so we can be sure streaming has stopped.
Fixes seeking back in time.
Something seems to have changed in libass that renders the old manual way of flushing events ineffective and libass then seems to ignore timestamps that are older than the ones last seen then if we do it the old way.
Fixes #916 (closed)