Windows (and possibly Linux?):
- The num lock key (which is used to set the current sample to 1 or 17) will
  also toggle the num lock state, which means that some of the numpad keys
  work like arrow keys instead. This makes the drumpad function useless! I
  used to have a Win32 API routine to fix this, but it seems to not work
  anymore after some SDL2 version, so I removed it. SDL2 seems to handle num
  lock internally these days, which ruins it.