Use installed shaders instead of embedded shaders

Having the default shaders squirrelled away in a resource file reduces
the user-hackability, which is a bad thing.

* Set up QFile search paths for shaders so that they can be found in a
  platform-independent way in the rest of the app

* Remove shader embedding compile option, since it doesn't make sense
  anymore.

* Kill off shader resource embedding, since we're not using it anymore.