Use all texture heaps in a fair way when textures need to be kicked in

Use all texture heaps in a fair way when textures need to be kicked in
order to make room for new textures. In particular this fixes texture
trashing on the first heap when the second heap is occupied by
currently unused textures (observed with Torcs and the Savage driver).

Heaps are weighted by their sizes by default but drivers can override
these and apply their own weights based on relative texture upload
speeds to the respective heaps.