Remove MetaRegion

In many places, MetaRegion was being used entirely internally, rather
than for gtk2/gtk3 compatibility. In these cases, it's simpler to just
depend on cairo-1.10 (for both gtk2 and gtk3) and use cairo_region_t.

The few places where we did need GDK compatibility (GdkEvent.region and
gdk_window_shape_combine_mask) are replaced with a combination of
converting GdkRegion to cairo_region_t and conditional code.

https://bugzilla.gnome.org/show_bug.cgi?id=632474