Add autoscroll when dragging past boundary of range

Add autoscroll when dragging past boundary of range

A problem with the zoom scroll mode is that you have to restart
if you hit the bottom of the screen before you hit the bottom
of your document.

This commit adds an autoscroll feature to the zoom scroll: if
you move outside the window while in zoom scroll mode, we keep
scrolling in the direction you were going until you let go
of the mouse button.

https://bugzilla.gnome.org/show_bug.cgi?id=704703