No way to set glClear/QSGEngine::setClearColor from QML in QtQuick2.0

There is no way to use glClear with a custom background colour from pure-QML.
A common pattern for a solid background colour is a Rectangle filling the root. But this is less efficient than setting the desired colour as the glClear value.

There should be a QML API or implementation for doing this, that maps a user set QML property to a QSGEngine::setClearColor.

If a background root-filling borderless rectangle with solid colour could be detected and translated to this, that might be the best option.
Alternatively there could be a method on the Qt object, or a global root colour property in the runtime.