WangFiller: Don't try to make changes outside of a fixed map

This avoids the problem, that tiles set outside of the map by the
algorithm will affect the tiles inside the map, which is not desirable.

Also, explicitly set tiles that are outside of the map to empty.  This
seems like a good idea, since it means the preview will still highlight
those areas, but it will not affect the map once applied.

src/tiled/wangbrush.cpp

@@ -586,10 +586,6 @@ void WangBrush::updateBrush()
         fill.region = completeRegion;
     }
 
-    // Don't try to make changes outside of a fixed map
-    if (!mapDocument()->map()->infinite())
-        fill.region &= currentLayer->rect();
-
     SharedTileLayer stamp = SharedTileLayer::create(QString(), 0, 0, 0, 0);
 
     wangFiller.setCorrectionsEnabled(true);

src/tiled/wangfiller.cpp

@@ -187,6 +187,22 @@ void WangFiller::apply(TileLayer &target)
     auto &grid = mFillRegion.grid;
     auto &region = mFillRegion.region;
 
+    // Don't try to make changes outside of a fixed map. Instead, to still
+    // provide some feedback in previews, explicitly mark this area as empty.
+    if (!mMapRenderer->map()->infinite()) {
+        const auto emptyRegion = region.subtracted(mBack.rect());
+
+        Cell empty;
+        empty.setChecked(true);
+
+        for (const QRect &rect : emptyRegion)
+            for (int y = rect.top(); y <= rect.bottom(); ++y)
+                for (int x = rect.left(); x <= rect.right(); ++x)
+                    target.setCell(x - target.x(), y - target.y(), empty);
+
+        region &= mBack.rect();
+    }
+
     if (!mCorrectionsEnabled) {
         // Set the Wang IDs at the border of the region to prefer the tiles in
         // the filled region to connect with those outside of it.