* The texture cache now acts more like a traditional cache,
  where the cache is a fixed size, and can evict items as required.
* The allocator is switched to a slab style allocator, allowing
  deterministic behavior when allocating and freeing.
* The slab allocator does not suffer from fragmentation like the
  previous allocator, and has no coalesce requirements.
* The eviction policy is a lot smarter than the previous implementation.
  Items are evicted from the cache as required, and not before.
* Since the shared cache is a fixed / bounded size, any allocations
  that are too large for the cache are created as standalone textures.
* In the rare / edge case of the fixed sized cache being full, and
  the cache not being able to evict items, these items also fallback
  to the standalone texture allocator.
* All textures using the sColor* samplers are now array textures.
  This will allow us to simplify the large image tiling code in
  the future.
* All texture updates are performed via PBO, to eliminate CPU
  driver stalls when updating textures.