Fixed terrain brush issue on staggered isometric maps

Neighbors can be two tiles away on a staggered map, which needs to be
taken into account when determining the bounds of the area to be
processed by the terrain brush.

Closes #1951

src/tiled/terrainbrush.cpp

@@ -408,16 +408,19 @@ void TerrainBrush::updateBrush(QPoint cursorPos, const QVector<QPoint> *list)
     QRect bounds;
     for (ConsiderationPoint point : transitionList)
         bounds |= QRect(point, point);
+
+    auto staggeredRenderer = dynamic_cast<StaggeredRenderer*>(mapDocument()->renderer());
+
     int margin = terrainTileset ? terrainTileset->maximumTerrainDistance() : 3;
+    if (staggeredRenderer)
+        ++margin;
     bounds.adjust(-margin, -margin, margin, margin);
 
     if (!mapDocument()->map()->infinite())
         bounds = bounds.intersected(currentLayer->rect().translated(-layerPosition));
 
     int initialTiles = transitionList.size();
 
-    auto staggeredRenderer = dynamic_cast<StaggeredRenderer*>(mapDocument()->renderer());
-
     // produce terrain with transitions using a simple, relative naive approach (considers each tile once, and doesn't allow re-consideration if selection was bad)
     while (!transitionList.isEmpty()) {
         // get the next point in the consideration list