wayland: implement the alpha-modifier protocol
The alpha modifier protocol allows clients to set a multiplier for the opacity of a surface, which allows them to offload some operations to KWin, which in turn may offload them to KMS in the future
Test client is weston-simple-egl at https://gitlab.freedesktop.org/Zamundaaa/weston/-/commits/work/alpha-mult
Protocol MR: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/287
Edited by Xaver Hugl