Add a convenience function for retrieving composition alpha state.

Retrieving whether composition is opaque is by itself likely to be more
useful to applications than also retrieving the colorization color.

For example, this value may be used to determine whether drawing a glow
behind text on glass like the OS does for the title bar, is appropriate.

Change-Id: Ic48a527727eae303f53831fa8524fdf02acaa2bd
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>