Log Message:
fasttracker2: update to b121

Changes since b120:

Beta #121 - 09.12.2018
- Completely rewrote how system requests (message boxes) work, which
  simplifies its usage a ton.
- Vsync now works properly in macOS Mojave again. How? Well, if you
  compile with XCode 9 on High Sierra instead of XCode 10 on Mojave,
  it just magically works!
- Fixed a couple of bugs with the "Delete/Rename" modes in Disk Op.
- Fixed some textbox routine bugs
- Fixed a bug with loop points when using the Mix function in Sample Ed.
- Fixed an off-by-one error in the y coordinate on sample hand-edit
- Waveform drawing: Fixed a rounding error on the negative part of the
  waveform while displaying a zoomed-out sample
- .pat files were accidentally not listed in instrument mode in Disk Op.
- Big code cleanup. Let me know if I made any new bugs, it's very possible!