wsi: Implement explicit sync for Wayland

Implements linux-drm-syncobj-v1 for compositors that support explicit sync.

Protocol: wayland/wayland-protocols!90 (merged)

Uses 2 timeline semaphores (syncobj) per-image, one for acq + release. Doing 1 timeline semaphore per-chain would suck given all the cross-queue heroics we'd need to pull off for no advantage. Separating acq + release into two timelines means we can be smarter in Acquire.