... or more exactly: Only use paint contexts with
gdk_cairo_draw_from_gl().

Instead of paint contexts being the only contexts who call swapBuffer(),
any context can be used for this, when it's used with
begin_frame()/end_frame().

This removes 2 features:

1. We no longer need a big sharing hierarchy. All contexts are now
   shared with gdk_display_get_gl_context().
2. There is no longer a difference between attached and non-attached
   contexts. All contexts work the same way.