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/opengl.h

@@ -59,6 +59,8 @@ glx_check_err_(session_t *ps, const char *func, int line) {
 }
 
 #define glx_check_err(ps) glx_check_err_(ps, __func__, __LINE__)
+#else
+#define glx_check_err(ps) ((void) 0)
 #endif
 
 /**