Only update CAD canvas item geometry when necessary

The call to setRect() in paint() handler was triggering canvas redraws
all the time, keeping one CPU core busy nearly all the time when
CAD dock was active.

src/gui/qgsadvanceddigitizingcanvasitem.cpp

@@ -42,7 +42,8 @@ void QgsAdvancedDigitizingCanvasItem::paint( QPainter* painter )
     return;
 
   QgsRectangle mapRect = mMapCanvas->extent();
-  setRect( mapRect );
+  if ( rect() != mapRect )
+    setRect( mapRect );
 
   int nPoints = mAdvancedDigitizingDockWidget->pointsCount();
   if ( !nPoints )