unstable: Add alpha-compositing protocol

It's based on the Chromium Wayland protocol of the same name ([1])
and Alexandros's modified version ([2])with a few changes
made to the blending_equation enumeration.

Add one more blending_equation "src_alpha / src_alpha"

[1] https://chromium.googlesource.com/chromium/src/+/master/third_party/wayland-protocols/unstable/alpha-compositing/alpha-compositing-unstable-v1.xml
[2] https://lists.freedesktop.org/archives/wayland-devel/2017-August/034741.html

Signed-off-by: Haihua Hu <jared.hu@nxp.com>

Conflicts:
	Makefile.am