After successful split, root-id masking in image layer does not update

[ceesem]

While doing a split, the image layer shows a gray mask to highlight the working root id. After doing a split, the selection updates with new root ids, but the masking does not. This means that unless you get rid of the split labels, you can't see the updated segmentation, adding an unnecessary and slow cursor-based user interaction. Given that the pychunkedgraph allows one to look up split locations as part of a root id's history, it seems like the correct behavior for the split tool is to reset its state after a successful split.