[PATCH wayland-protocols v3] Add alpha-compositing protocol

[Simon Ser]

> This protocol specifies a set of interfaces used to control the alpha
> compositing and blending of surface contents, based on the Chromium
> Wayland protocol of the same name [1].
>
> Signed-off-by: Scott Anderson <scott.anderson at collabora.com>

This version LGTM.

Reviewed-by: Simon Ser <contact at emersion.fr>

Thanks!