This includes a copy of the diff(1) algorithm used by git diff by Davide
Libenzi.
It's used for the common case ofcontainer nodes having only very few
changes for the few nodes of child widgets that changed (like a button
lighting up when hilighted or a spinning spinner).
This is now tracking the clips added by the clip nodes.
If any particular node can't deal with a clip, it falls back to Cairo
rendering. But if it can, it will render it directly.
