Qt Wayland Compositor QML API doesn't build without opengl

The compositor QML API doesn't build without OpenGL at the moment. It used to do this a couple of years ago, so some infrastructure is obviously in place, but a lot of the new code and refactoring has been assuming OpenGL is available.

Running a compositor without OpenGL can be useful on devices where OpenGL isn't supported. Or if GPU resources should be reserved/prioritized for some specific purposes.