backend: try gdk backend before x11/wayland/egl

Quite a few people at Guadec complained of pinpoint being broken in
speaker+fullscreen mode, with slides being half displayed. It turns
out that the X11 backend of clutter was being used and this backend
assumes the size of the current monitor is the size of the X screen
(that's not the case with multiple monitors).

To work around the shortcomings of the X11 backend we should probably
position the GDK one before. GDK implements most of the logic the
ClutterStage needs and is probably more tested.

https://bugzilla.gnome.org/show_bug.cgi?id=734587