[cube] transparent cube

[mbx162]

Wayfire should be able to configure the cube transparency.

Instead of a background image the desktop should display the skydome/cubemap permanently, making it possible for the user to peek through the free space on the desktop on the opposite side of the cube

The user configures cube.background=#1A00000A, wayfire takes the alpha (0A) of the background to make the cube-surface transparent.