Fix backjumping on seeded grids

Currently, the backjumping algorithm jumps back to the most recently filled slot that crosses the failed slot. However, this seems to cause issues, especially in seeded grids. The slot-choosing heuristic can basically partition the grid into multiple unrelated sections, and then the backjumping algorithm can sometimes bounce back and forth between these sections, undoing them repeatedly for no reason. This might be an inherent flaw with backjumping (and might require some dynamic approach), but there is probably some improvement to the current algorithm that would make it better.

e.g. try filling the 15xjja grid with MinlookBackjumpFiller. It fills small words in the middle first, then tends to switch off repeatedly between different corners, undoing each other because they were filled more recently than the small words in the middle.