gdk: Add function to retrieve the GdkVisual

Straight from Cogl.

This allows us to propagate the GdkVisual Cogl and Clutter use to
embedding toolkits, like GTK+.

The function is annotated as being added to the 1.22 development
cycle because it will be backported to the stable branch, so that
downstream developers can package up a version of Clutter that does
not crash on nVidia.

https://bugzilla.gnome.org/show_bug.cgi?id=747489
(cherry picked from commit 2d5b5aa8

)
Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>