Add `Color.toBytes` to match `Color.fromBytes` (and also to make tests more readable)
Add `Texture.clear`.

Also fill in more documentation, and clean up the code and the specs for Texture.
Fix logo processing.

* make logoOffset work properly
* clear texture before copying in new logo regions
Remove `Texture.clear` and instead just delete and recreate the texture every time it changes.

texImage2D doesn't work reliably with a null source, and apparently I'm crazy for wanting to clear a texture anyway.
Move sampler-related specs to a separate spec.
Merge pull request #507 from AnalyticalGraphicsInc/logo-offset

Fix problems with logos