Add explicit synchronization, with timelines

This PR implements explicit synchronization. Unlike previous attempts, it uses a timeline abstraction based on drm_syncobj like Vulkan.

To inter-operate with other APIs (KMS, EGL, linux-explicit-synchronization-v1), the timeline points can be converted from/to sync_file FDs.

Test with: build/examples/explicit-sync -s weston-simple-dmabuf-egl. Use WAYLAND_DEBUG=server to check that fences are exchanged. To test the DMA-BUF sync_file extraction, use weston-simple-egl as test client (requires kernel patch).

TODO:

• Write an example compositor
• Extract sync_file from DMA-BUF when client doesn't support linux-explicit-synchronization-v1
• Add wlr_texture upload synchronization
• Figure out if this API is okay for Vulkan
• Detect support in backends and renderers
• Add support for EGL_EXT_image_implicit_sync_control (Mesa support still missing, but let's create a good excuse for them to add it)
• 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
• Output cursors let's wait until output layers for this

Previous work:

• https://github.com/swaywm/wlroots/pull/2070
• https://github.com/swaywm/wlroots/pull/1685

Future work:

• Multi-GPU support in the DRM backend
• Wayland backend support
• Vulkan renderer support