mpegplayer: Use the core DSP to process audio. Removes the sample rate restriction...

mpegplayer: Use the core DSP to process audio. Removes the sample rate restriction on audio and any mpeg audio samplerate may be used. Use the global sound settings for audio output with the option to force any one of the processing stages off. All are forced off by default. I didn't personally care to fully duplicate the Sound Settings menu which would have been needed since using the core one would affect settings globally and exactly the same configuration probably isn't desired since the CPU load for video playback is much greater. Rebalance the threading to compensate with some expense to buffering speed.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@16194 a1c6a512-1295-4272-9138-f99709370657