This test renders a cube against a cleared black background.

The cube should be able to be manipulated by dragging the mouse
pointer around the window.

The aim is to have the simplest possible test which shows rendering
3D content with no 2D content.

When the 3D content is free of 2D QML in this way optimized approaches may
be used and this test (along with "background") is meant to be the simplest test
of this behavior.
