Fix what I believe is a bug introduced while porting

lib/wfc/model.dart

@@ -108,7 +108,7 @@ abstract class Model {
   }
 
   int _nextUnobservedNode(Random random) {
-    if (_heuristic == Heuristic.Entropy) {
+    if (_heuristic == Heuristic.Scanline) {
       for (var i = _observedSoFar; i < _wave.length; i++) {
         if (!_periodic && (i % cMx + cN > cMx || i ~/ cMx + cN > cMy)) {
           continue;