Before, async video sources would flicker because they were only being
drawn when they were updated.  So when updated, they'd draw that frame,
then it would stop drawing it until it updated again.  This fixes that
issue and they should now draw properly.

Also, fix a few other minor bugs and issues relating to async video,
and make it so that non-async video filters can be properly applied to
them.

For the purposes of testing, change the 'test-random' source to an async
video source that updates every quarter of a second with a new random
face.

Also fix a bug where non-async video sources wouldn't have filter
effects applied properly.