Improve the algorithm for proportional mouse tracking

Change the proportional algorithm so stop moving the zoom region
when cursor is in a "padding region" at the edge of the screen.
(The padding region is a 10th of the screen at 2x zoom, and smaller
for higher zooms.)

Based on earlier versions from Jon McCann and Florian Muellner.

https://bugzilla.gnome.org/show_bug.cgi?id=629950