Fix jumping map in canvas

This would happen only with overview canvas visible. Found by Mathieu.

Replacing the forced repaint() call by more suitable update()

src/gui/qgsmapoverviewcanvas.cpp

@@ -49,7 +49,7 @@ class QgsPanningWidget : public QWidget
       if ( p == mPoly ) return;
       mPoly = p;
       setGeometry( p.boundingRect() );
-      repaint();
+      update();
     }