- Add a way to generate a FasTCConfig.cmake file so that you can
use cmake without having to install it.
- Add install paths for users that want to install it.
- Hide all public headers in FasTC/ qualified include path, this way we
know what files are public directly from the source. Also, it lets us
define build-tree and install-tree include directories a lot easier.
1. Avoid the need for multiple-of-four input textures. If you don't pass
a multiple of four then we'll do our best to pad the image.
2. Fix a bunch of bugs where we didn't accurately report why we couldn't
compress a texture and just crashed instead.
3. Some code refactoring to make certain if statements more readable.