Track the size in memory of the texture cache.

The texture cache was previously checking the number of stored textures. This was
not very useful as this could easily lead to an abuse of memory. The new cache
instead tracks the total size occupied in RAM by the cached textures. When a new
texture is generated, older textures are kicked out as needed.

Change-Id: Ib27142f4a018d5bf84774c1fb6f45a67a85f20bc