Change the way the update area is tracked during paint

Change the way the update area is tracked during paint

First of all we track the current update area during an
update in window->active_update_area. This will be used later
in end_paint to know the damaged area.

Secondly we keep track of old update areas for the last 2
frames. This will later allow us to reuse old framebuffer
contents in double or tripple buffer setups, only painting
what has changed since then.