Make the ResourcePool hold FBOs as a per-context resource.

This is an attempt to get out of the FBO sharability mess (unfortunately
we can't just stop having persistent FBOs, due to NVidia performance).
We now require the client to tell us whenever a context is going away,
and we try to be more careful about not deleting them in the wrong context.

Also, we assumed FBO names were globally unique, which isn't necessarily
true, so re-key them.

For good measure, we were deleting FBOs off the freelist from the front,
not the back as we should have -- fixed.