When disposing a GdkDrawingContext we should unset the association
between the instance and the Cairo context; this avoids stale pointers
in case a reference that has acquired on the Cairo context survives the
lifetime of the GdkDrawingContext.
Instead of associating the GdkWindow that created the GdkDrawingContext
we can directly bind the Cairo context to the GDK drawing context.
Cairo contexts created via gdk_cairo_create() go back to not having a
GdkWindow associated to them, like they did before we introduced the
gdk_window_begin_draw_frame() API.
https://bugzilla.gnome.org/show_bug.cgi?id=766675
Instead of giving out Cairo contexts, GdkWindow should provide a
"drawing context", which can then create Cairo contexts on demand; this
allows us to future proof the API for when we're going to use a
different rendering pipeline, like OpenGL.
https://bugzilla.gnome.org/show_bug.cgi?id=766675
