[Cogl] Expose callback v2

[Neil Roberts]

I discussed the expose callback patches with Robert a bit over IRC and
we decided to make the following changes which this new patchset covers:

• It is now called a ‘dirty callback’ instead of an expose callback.
  The concept of expose seems a bit out of place in the world of
  compositors which don't make the window dirty when a window is
  exposed.

• The simulated dirty events are now sent when the window is first
  allocated instead of every time it is shown because on Wayland the
  contents of the surface won't be lost when it is hidden so there is
  no need to paint again.

• There is now a patch to make the WGL winsys automatically retrieve
  Windows messages so it can now handle the WM_PAINT messages
  correctly.

• The cogl-hello example queues an idle handler to do the actual
  redraw so that if Cogl sends multiple dirty events it won't
  redundantly draw multiple frames.

Regards,
- Neil