Correctly implement refresh synchronization fences

Correctly implement refresh synchronization fences

* configure.ac (HAVE_XSYNC): Also check for XSyncTriggerFence.

* src/xfns.c (Fx_create_frame): Create fences.
* src/xterm.c (x_atom_refs): New atom.
(x_sync_trigger_fence, x_sync_init_fences, x_sync_free_fences):
New functions.
(x_sync_update_finish): Trigger the appropriate fence.
(x_free_frame_resources): Free fences.
* src/xterm.h (struct x_display_info): New atom
`_NET_WM_SYNC_FENCES'.
(struct x_output): New field `sync_fences'.