Compass tool: draw center cross for each new arc

This solves an issue where drawing several arcs in a row resulted in
only the first one being saved.

src/tools/UBGraphicsCompass.cpp

@@ -308,6 +308,7 @@ void UBGraphicsCompass::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
     {
         updateResizeCursor();
         updateDrawCursor();
+        mDrewCenterCross = false;
         event->accept();
     }
     else if (closeButtonRect().contains(event->pos()))