renderer: Assert having been unrealized in dispose

renderer: Assert having been unrealized in dispose

We cannot unrealize a renderer in the dispose function, because that
would cause this chain to happen:
  gsk_gl_renderer_dispose
  gsk_renderer_dispose
  gsk_renderer_unrealize
  gsk_gl_renderer_unrealize
So we would call into thje GL renderers unrealize when it has already
(partially) disposed itself and ause accesses to dead variables.