ext-session-lock-v1: new protocol

This protocol allows for a privileged Wayland client to lock the session and display arbitrary graphics while the session is locked.

The client is responsible for performing authentication and informing the compositor when the session should be unlocked. If the client dies while the session is locked the session remains locked, possibly permanently depending on compositor policy.

This takes a different approach from !100 (closed), leaving more details up to the compositor for flexibility.

I intend to implement this protocol in river as river-lock-v1 and write a client in the next few weeks. I decided to submit it here as well though as I believe other compositor authors are also interested in a better way to implement screenlocking. Seems that wayland-protocols might be moving a bit faster than expected :)

• Reviewer: @emersion
• Compositor impl: wlroots/wlroots!3414 (merged), https://github.com/riverwm/river/pull/497
• Client impl: https://github.com/swaywm/swaylock/pull/219