Fix problems with logos

* Setting logoOffset works properly again (fixes #361)
* Logos changing no longer leaves behind old pixels (see Imagery Manipulation sandcastle example)
* Add `Color.toBytes` to match `Color.fromBytes` for more convenient pixel color checking in specs
* Clean up Texture and add some more documentation for the things that I know how to document.
   * (I was in there because I was going to add a Texture.clear but that didn't work right, so I took that out)
* Clean up Texture specs too.