Refactor the XSync fence code

Use a temporary fence everytime. Convert the Xlib XSync functions to use
xcb_sync_*.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/win.h

@@ -3,8 +3,6 @@
 // Copyright (c) 2013 Richard Grenville <pyxlcy@gmail.com>
 #pragma once
 #include <stdbool.h>
-#include <X11/Xlib.h>
-#include <X11/extensions/sync.h>
 #include <xcb/damage.h>
 
 // FIXME shouldn't need this
@@ -98,8 +96,6 @@ struct win {
   winmode_t mode;
   /// Whether the window has been damaged at least once.
   bool ever_damaged;
-  /// X Sync fence of drawable.
-  XSyncFence fence;
   /// Whether the window was damaged after last paint.
   bool pixmap_damaged;
   /// Damage of the window.