- Updated dr_mp3 and implemented it
- Reviewed sampleCount vs frameCount
- Reviewed XM playing (some weird things...)

1. Always synthesize a complete frame of audio, using a second buffer (this prevents gaps in playback)
2. Sine is computed correctly, with an adjustable frequency
3. User can modulate frequency in real-time with mouse
4. Entire audio buffer data is shown, visually demonstrating how sine changes in wavelength