Associate the drawing context to the Cairo context

Associate the drawing context to the Cairo context

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