Update to Text elements causes slow down in QtQuick UI

Updates to the content of the Text elements in an application will slow down the operation of other parts of the application. The problem gets worse when there are lots of QML Elements visible.

The problem is actually difficult to see on a powerful Desktop machine, but quite significant on embedded devices such as iMX53 or iMX6. The attached sample application shows the issue - you should see the ‘Label 4’ counter pause regularly while the application is running, although the result is a lot more obvious on an embedded device.

Investigation into the problem shows changing the text in a Text element causes the existing GlyphNodes to be removed and new ones added.

GlyphNodes are subclasses of the GeometryNode, and adding a GeometryNode without a root to the SceneGraph causes it to rebuild itself from the bottom up.

Setting clip: true on the Text element stops the fullRebuild from happening, but slows down the application more than not having clip: true.