* Sort texture cache items into bins in order to minimize the free list
  scan time.

* Improve the texture cache API by switching the "alternate" Boolean to
  an enum.

* Store texture sizes in cache items. This will be useful when we switch
  render targets to be allocated out of the texture cache.

* When coalescing rects, don't eagerly iterate to a fixed point. Instead
  just do as many iterations as we need to satisfy the request.