Atomic output updates

[emersion]

This would let us:

• Enable and disable multiple outputs without the fear of hitting the CRTC limit
• Test configurations before applying them
• Rollback if something fails
• More cool stuff

https://github.com/swaywm/wlr-protocols/pull/38#issuecomment-470472049

---

wlroots has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/1640

[emersion]

API proposal:

1. Make wlr_output_set_* functions double-buffered
2. Add wlr_output_commit to apply the pending state
3. Add wlr_backend_commit to apply the pending state for all outputs. Backends can choose not to implement this, in which case the function will fallback to calling wlr_output_commit on each output
4. Add wlr_output_test to check if the pending state is valid without applying it
5. Deprecate non-atomic events (enable, mode, scale, transform)

[emersion]

Items 1, 2 and 4 are now done.