Use FFMPEG to decode Atrac3+ audio, thanks to Maxim's new decoder patch.

This gets rid of the plugin loading support entirely, and instead uses FFMPEG directly to decode Atrac3+ audio.

Now, PPSSPP will work with AT3+ music out of the box, no plugin necessary.

Needs lots and lots of testing before we merge, I would love to get this in before I push 0.9.5.