Low-CPU usage mode for replay gain scanning

If you're running groovebasin on a computer with limited CPU power (or you want to keep groovebasin's CPU usage unobtrusive to the system), there should be a mode that only does just-in-time replay gain scanning. That means when we're already decoding the song in order to play it (or re-encode it for streaming), we use the decoded data to do replay gain scanning, and we adjust the loudness live. This way we get replay gain without any extra decoding CPU usage.