audio/fasttracker2: Update to v1.12

Changes since v1.11:

v1.12 - 13.03.2020
- Bugfix: Some broken XMs/XIs have a non-sense value in the "mute" instrument
  field. This is now clamped correctly so that such modules and instruments
  will not yield muted sound. This fix was actually present earlier, but it was
  broken again some time ago. You will very rarely bump into this issue anyway.
- Micro-optimization to how voice/ramp volumes are calculated (4x DIVs removed)
- Windows/macOS: SDL was upgraded from 2.0.10 to 2.0.12

(fox)