mesa/gl: Sync with Khronos registry

mesa/gl: Sync with Khronos registry

Update GL headers and xml API from upstream Khronos registry (commit
3d0c3eb). Keep `BUILDING_MESA` quirk in glext.h.

mesa/extensions: Expose EXT_EGL_sync instead of MESA_EGL_sync to reflect
Khronos request of changing this extension's scope from MESA to EXT.
EGL_EGL_sync is also the name of the extension that has been merged into
the upstream Khronos GL registry.

Remove MESA_EGL_sync spec txt from Mesa tree as it is now published as
EXT by Khronos.

v1: Remove MESA_EGL_sync spec and squash commits (Eric E)

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>