diff: Allow aborting a diff

diff: Allow aborting a diff

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.