scroll-view-fade: Do the offset math in the shader

scroll-view-fade: Do the offset math in the shader

This doesn't (or shouldn't) change the visual appearance of the fade
effect, but does do all the testing math inside the shader, rather
than on the CPU. This will make fading the offset much easier in
the future.

https://bugzilla.gnome.org/show_bug.cgi?id=689249