wip: backend/drm: add libliftoff interface

TODO:

- Make it work on multi-output setups
- Make it handle VRR and gamma

backend/drm/meson.build

@@ -1,12 +1,16 @@
+libliftoff = dependency('libliftoff', version: '>=0.2.0', fallback: ['libliftoff', 'liftoff'])
+
 wlr_files += files(
 	'atomic.c',
 	'backend.c',
 	'cvt.c',
 	'drm.c',
 	'legacy.c',
+	'libliftoff.c',
 	'properties.c',
 	'renderer.c',
 	'util.c',
 )
 
 features += { 'drm-backend': true }
+wlr_deps += libliftoff