When the max cost for finding a path gets to high, the diff can now be
aborted.
Because render nodes have a fallback method (by just marking the whole
bounds of the nodes as different), we use this to improve performance
of diffs.
This brings fishbowl (which is basically a container node with N images
that change every frame) back to close to previous performance.
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).
