Help: Fix font point sizes with litehtml

Change-Id: Ia2d7434ed965b00075747c0bdd43d90a236b99dd
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/help/qlitehtml/qlitehtmlwidget.cpp

@@ -422,6 +422,7 @@ QUrl QLiteHtmlWidget::url() const
 
 void QLiteHtmlWidget::setHtml(const QString &content)
 {
+    d->documentContainer.setPaintDevice(viewport());
     d->documentContainer.setDocument(content.toUtf8(), &d->context);
     verticalScrollBar()->setValue(0);
     horizontalScrollBar()->setValue(0);