[layouts] Avoid temporary rendering artefacts when grid properties ch…

…ange

There's no need for the Qt item caching here, as map items have
their own internal cache handling.

(cherry picked from commit fd881db)

src/core/layout/qgslayoutitemmap.cpp

@@ -43,6 +43,8 @@ QgsLayoutItemMap::QgsLayoutItemMap( QgsLayout *layout )
 
   assignFreeId();
 
+  setCacheMode( QGraphicsItem::NoCache );
+
   connect( this, &QgsLayoutItem::sizePositionChanged, this, [ = ]
   {
     shapeChanged();