Enable child widgets (without a native window) that render to an FBO
and are composed with the raster backingstore by the platform plugin.
A preliminary version of QOpenGLWidget is included as private API.

Change-Id: I8f984a4d7db285069ce3d6564707942c823d890d
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>