Patch Heal/Clone/Erase

When healing or cloning, I often need to extend a texture up to a straight edge. At present the clone and heal brushes are round and imprecise, but the ai eraser won't erase inside a clone/heal layer, so either allow straight edge patches for such texture heal/cloning, or allow the ai eraser to erase inside a heal/clone layer, whichever is easiest to implement.