Ignore pieces with only one state during solving. (#1323)

Straight up dumb off-by-one error. Oops!

This should make the solver a little faster (though it mostly uses the
piece state map to look up states, so the shorter piece list won't have
a huge effect).

lib/src/back_end/solver.dart

@@ -42,7 +42,7 @@ class Solver {
 
     void traverse(Piece piece) {
       // We don't need to worry about selecting pieces that have only one state.
-      if (piece.states.isNotEmpty) unsolvedPieces.add(piece);
+      if (piece.states.length > 1) unsolvedPieces.add(piece);
       piece.forEachChild(traverse);
     }