Reuse tile textures when appropriate

Instead of constantly destroying, recreating, and binding buffers to
textures, we should reuse tile textures when the buffer does not change.
Create textures immediately before compositing

This step is fairly cheap and waiting until the last minute, prevents
us from creating two textures when content changes twice between
composite passes.
Merge pull request #87 from mrobinson/cleanup

A few cleanups for Tile Texture creation