Make QuickSync surfaces be allocated dynamically.

Make QuickSync surfaces be allocated dynamically.

Instead of specifying that frame N always uses surface N % 16,
we allocate dynamically from a pool. This both makes a lot more
sense, and also allows us to hold onto surfaces for other reasons
(like that we want to render _from_ them) in a future patch.

This also necessitated explicit usage tracking of reference frames
in order to avoid display corruption (you can't reuse a surface
before its dependent frames are also done rendering); I'm unsure
if this actually was correct before, but it's possible that the
implicit serialization made sure it actually was, because I've
run the existing code pretty hard before without seeing reference
frame corruption.