staging: add alpha-modifier protocol

This protocol allows clients to set an alpha multiplier for the whole surface, which allows it to offload alpha changes for the whole surface to the compositor, which in turn can offload them to KMS.

Signed-off-by: Xaver Hugl xaver.hugl@kde.org

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• Review (@emersion)
• 3 ACKs
  • wlroots (@emersion)
  • KDE (@zzag)
  • Smithay/Cosmic (@Drakulix)
  • Weston (@pq)
• 3 implementations
  • KWin: https://invent.kde.org/plasma/kwin/-/merge_requests/5462
  • wlroots: wlroots/wlroots!4616
  • gamescope client: https://github.com/ValveSoftware/gamescope/tree/alpha-modifier