Fix issues from recent glyph caching change

There were 2 issues remaining after a recent change to support
glyph caching from multiple textures:
- memory in the GPU for all textures was being allocated automatically.
This is now lazy, being allocated only when those textures are first
needed.
- filtering (applied when a rendered object is transformed) was ignoring
the new multiple-texture structure. Filtering should be applied correctly
whenever we change textures.

Change-Id: I5c8eb8d46c73cd01782a353fc79b11cacc2146ab