PP: Less/smarter lookahead for Healing Clone tool

Since its inception, the Healing Clone tool seems to sample in the direction of brush travel farther than it really needs to, often resurrecting hard-edged ghosts of what it's gone over (even at 100% opacity and with multiple strokes) as though it's not updating completely. Additionally, it will sample beyond the image/page limit into inky blackness which it coughs up into the path of the tool, making its use near edges a real pain. I like Healing Clone, it's one of the better newer tools in PP's arsenal, and it's doing something new and exciting that none of the legacy tools could really do, but it needs encouragement and improvement!