Draft: Add DRI3/Present protocol for explicit synchronization

Hi, just send out this rough change to illustrate a way to explicit sync. Implementing it require a bunch of work, so I need communities' feedback at the begging to make sure it's the right way.

The key idea is using timeline syncobj as the wait and idle fence of present protocol. The reason to use timeline syncobj instead of sync_file is:

1. it's GPU driver friendly, as most GPU use syncobj for command submission
2. light weight, send FD once then use raw number afterwards
3. may be required by some GPU which support user space command stream synchronization

Some guys (like @jekstrand) also mentioned similar way before, so I'm just the one who plan to implement it for X11 world.

Any feedback is welcome, thanks.

CC: @ajax @daenzer @daniels @airlied @danvet

Using timeline drm_syncobj sounds good to me. Otherwise no opinion since I'm very far away from ever having worked on X11 protocols.

Thanks for the feedback. Seems X11 dev topic gets little attention nowadays.

Indeed it does; virtually all work nowadays is on the transition to Wayland.

This is not very clear and should be expanded upon.