security-context: new protocol

This is a variation of the unveil protocol I suggested in the Weston issue about security contexts. This lets sandbox engines such as Flatpak attach a security context to sandboxed clients. The compositor can then restrict which features are made available to that client.

Signed-off-by: Simon Ser contact@emersion.fr
References: weston#206 (closed)

• Review: @jadahl
• ACKs
  • wlroots (@emersion)
  • Mutter (@jadahl)
  • Smithay (@Drakulix)
• Implementations:
  • wlroots: wlroots/wlroots!3589 (merged)
  • Flatpak: https://github.com/flatpak/flatpak/pull/4920
  • crosvm: https://puck.moe/git/crosvm/commit/?h=wayland-security-context&id=dbdba0bf64517181e30402882d36e0cbcd4934bd