Bug fix #181: Add --xrender-sync{,-fence}

- Add --xrender-sync{,-fence} to deal with redraw lag issue on GLX
  backend. --xrender-sync-fence requires a sufficiently new xorg-server
  and libXext. NO_XSYNC=1 may be used to disable it at compile time.
  Thanks to tchebb for reporting and everybody else for testing. (#181)

- A bit code clean-up. Replace a few XSync() with XFlush() to minimize
  the latency.

src/compton.h

@@ -275,6 +275,15 @@ free_paint(session_t *ps, paint_t *ppaint) {
   free_pixmap(ps, &ppaint->pixmap);
 }
 
+/**
+ * Free w->paint.
+ */
+static inline void
+free_wpaint(session_t *ps, win *w) {
+  free_paint(ps, &w->paint);
+  free_fence(ps, &w->fence);
+}
+
 /**
  * Destroy all resources in a <code>struct _win</code>.
  */
@@ -883,7 +892,7 @@ static void
 damage_win(session_t *ps, XDamageNotifyEvent *de);
 
 static int
-error(Display *dpy, XErrorEvent *ev);
+xerror(Display *dpy, XErrorEvent *ev);
 
 static void
 expose_root(session_t *ps, XRectangle *rects, int nrects);