Straightforward port of geometrictransform plugin
https://bugzilla.gnome.org/show_bug.cgi?id=625908
Ports gleffects "square" filter to geometrictransform. Maps a region around the center into a zoomed square and smoothly get back to normal zoom. With faces it makes a funny "cube-face" effect. https://bugzilla.gnome.org/show_bug.cgi?id=625722