Basic cleanup of what we're doing already. This moves a bunch of the "inside the fade area" checks to the top, before doing any calculations. https://bugzilla.gnome.org/show_bug.cgi?id=689249
This doesn't make any changes to the shader outside of adding a header; it simply makes changes in the future easier to edit. https://bugzilla.gnome.org/show_bug.cgi?id=689249