sync from mesa

Initial repo re-org, and migration to meson, to enable syncing changes back from mesa.

https://gitlab.freedesktop.org/robclark/envytools/-/commits/syncit/ has the results of running the script to sync to current mesa. Until this MR lands I'll be regenerating it and force-pushing. After that the rough plan is to have the script automatically update the syncit branch and then manually pull that over. If that is reliable for some time, I might switch it to push directly to master.

Note that CI will initially fail until we sync over some changes from mesa.