This commit is in preparation for the work happening in GTK3, which will
use Cairo for drawing exclusively. So it is necessary to move all
drawing code to Cairo. In this commit the "gtk2" code is used for both
gtk2 and gtk3; compatibility with newer versions of gtk3 where different
code is needed will be added subsequently.
For compatibility with older GTK versions, the file gdk2-drawing-utils.h
provides a compatibility layer.
The commit changes the API of libmutter-private.
https://bugzilla.gnome.org/show_bug.cgi?id=630203
aa65f94c67