Slant: use the new findloop for loop detection.

Slant: use the new findloop for loop detection.

The old face-dsf based loop detector is gone, and now we just call
findloop instead.

This is just a code cleanup: it doesn't fix any bugs that I know of.
In principle, it provides the same futureproofing we gained by making
the same change in Net, but Slant as a puzzle is less adaptable to
topologically interesting surfaces - in particular, you _can't_ play
it on any edgeless surface like a torus or Klein bottle, because no
filled grid can be loop-free in the first place. (The only way a
connected component can avoid having a loop surrounding it is if it
connects to the grid edge, so there has to _be_ a grid edge.) But you
could play Slant on a Mobius strip, I think, so perhaps one day...