Increase subtitle toggle speed.

This pull request addresses [#3650](https://trac.mpc-hc.org/ticket/3650). Turning subtitles on takes up to 3 seconds sometimes on my Intel Atom netbook. It's not fast enough to toggle subs on the fly.

Unfortunately, I'm not that familiar of MPC-HC internals for now. This patch works for me, but I'm not sure I didn't insert a bug. So, its more RFC than actual pull request.