Changes 6.9.74 -> 6.9.75:
* GUI: Tweak default background color of the editor
  and the modular mixer.
* Audio/MIDI: Fix erroneous error message
  ("RT_free failed") possibly being displayed when
  sending and receing MIDI from VST plugins. Bug
  probably introduced around 6.9.66.
* Sequencer: Warn about non-linear mapping if
  loading an older song that has Crossover
  automation for the sampler instrument.
* Editor: Warn about non-linear mapping if loading
  an older song that has Crossover automation for
  the sampler instrument.
* Mixer: Made it possible to configure the
  background color of the modular mixer separately.
* Sampler instrument: Fix loading songs with custom
  loops.
* API: Fix wrong error message possibly being
  displayed when sending/receiving MIDI.