Sprite handling framework

[Jesse Barnes]

Ok these need some actual review.  I have things working here (you can
test on a current kernel by enabling the DEBUG_PLANES code) and I think
the infrastructure is starting to look ok, especially now that I don't
leak fbs on every assignment. :)

The new buffer reference code makes things a bit nicer, but they could
probably still be easier to follow, so suggestions are welcome there.

I'm also definitely open to suggestions on how to split things more
finely to make the patches easier to digest...

Thanks,
Jesse