Audio mixer

[sornas]

• [x] Gain (#39)
• [x] Pitch (#42)
• [x] Gain variance (#40)
• [x] Pitch variance (#42)
• [x] Master gain (#43)
• [ ] Limiter

[sornas]

I asked a friend of mine about limiters and it seems like a good limiter does more than just clipping. My idea about sigmoid squashing would be just barely better than hard clipping and they said that we could for example take the average amplitude over a larger-than-our-current-buffer timespan which sounds kinda statefull and not what we should focus on right now. Also, hard clipping seems to be what SDL (or the audio device) does anyway so we shouldn't need to do anything.