The last video frame is another case that has a separate code path,
although it's pretty similar to the one in commit 73e5aa87. Fix this
in a different way, which also takes care of the last frame case,
although without context the code becomes slightly more tricky.
As further cleanup, move the decision about framedropping itself to
the same place, so the check in vo.c becomes much simpler. The check
for the vo->driver->encode flag, which is remvoed completely, was
redundant too.
Fixes#1480.
126b7c4739