Add explicit synchronization, with timelines, take 2

Same as !3282 (closed) but rebased and reworked for the new rendering API.

TODO:

• Detect support in backends and renderers
• Add support for cached state to linux-explicit-synchronization-v1
• Documentation
• Sanity checks (e.g. in wlr_output_commit)
• Consider ref'counting wlr_render_timeline

Previous work:

• !3282 (closed)
• https://github.com/swaywm/wlroots/pull/2070
• https://github.com/swaywm/wlroots/pull/1685

Future work:

• Multi-GPU support in the DRM backend
• Hardware cursor support
• Output layers support
• Wayland backend support
• Vulkan renderer support
• linux-explicit-synchronization-v1 (!4574 (closed))
• linux-drm-syncobj-v1 (!4262 (merged))