Using XFlush() instead of XSync() avoids a round trip to the X server

on each call to DevUpdate().  That speeds plotting when using XQuartz
on Macintosh and makes iplot useable there.

src/frontend/plotting/x11.c

@@ -894,7 +894,7 @@ int
 X11_Update(void)
 {
     if (X11_Open)
-        XSync(display, 0);
+        XFlush(display);
     return 0;
 }