Avoid creating multiple polygon points at the same location

When creating a polygon or polyline, it was easy to end up with multiple
points placed at the same location. Since this is basically never
intended, additional clicks on the same location will now be ignored.

Thanks to @Aranda for pointing out this issue and suggesting this fix.

src/tiled/createobjecttool.cpp

@@ -179,13 +179,19 @@ void CreateObjectTool::mousePressed(QGraphicsSceneMouseEvent *event)
                 else
                     cancelNewMapObject();
             } else if (event->button() == Qt::LeftButton) {
+                QPolygonF current = mNewMapObjectItem->mapObject()->polygon();
+                QPolygonF next = mOverlayPolygonObject->polygon();
+
+                // If the last position is still the same, ignore the click
+                if (next.last() == current.last())
+                    return;
+
                 // Assign current overlay polygon to the new object
-                QPolygonF polygon = mOverlayPolygonObject->polygon();
-                mNewMapObjectItem->setPolygon(polygon);
+                mNewMapObjectItem->setPolygon(next);
 
                 // Add a new editable point to the overlay
-                polygon.append(polygon.last());
-                mOverlayPolygonItem->setPolygon(polygon);
+                next.append(next.last());
+                mOverlayPolygonItem->setPolygon(next);
             }
             break;
         }