Straightforward port of geometrictransform plugin
Add a "mode" enum property to mirror that defines how to split the frame and with side reflect. https://bugzilla.gnome.org/show_bug.cgi?id=625908
Ports gleffects "mirror" filter to geometrictransform. Simple yet effective mirror effect, splits the image into halves and reflect the first into the second. https://bugzilla.gnome.org/show_bug.cgi?id=625722