assrender: fix seeking backwards

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