Call QPainter::end() prior to QPixmapCache::insert()

Inserting a QPixmap with active painters on it causes it to
do a deep copy of it, which is unnecessary.

Task-number: QTBUG-58653
Change-Id: I49123ffcec2e12a01c87974b7a513d68a11185d4
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/stylehelper.cpp

@@ -455,6 +455,7 @@ void StyleHelper::drawIconWithShadow(const QIcon &icon, const QRect &rect,
 
         // Draw the actual pixmap...
         cachePainter.drawPixmap(QRect(QPoint(radius, radius) + offset, QSize(px.width(), px.height())), px);
+        cachePainter.end();
         cache.setDevicePixelRatio(devicePixelRatio);
         QPixmapCache::insert(pixmapName, cache);
     }