gtktiledviewer: Show 4 tiles at varying scale factors. DON'T MERGE

I.e. subdivide the document into 4 tiles: one at 100% scaling,
one at 200%, one at 50%, one at 25% -- these are then post-scaled
(in gdk) and assembled to show as one document again.

This is only for test purposes, and shouldnt go into master. Could probably
be useful as a separate test widget though.

Change-Id: Id26a5418d88ce17525358d27d313cf9884fa1494