[FR]: Add a simple way to render an Edit on a background thread

[drowaudio]

• [x] High level API that takes an Renderer::Parameters and renders an Edit on a background thread
• [x] Returns immediately with a handle
• [x] Runs on a an internal background thread
• [x] Takes a std::function that it calls on completion
• [x] Handle:
  • [x] Progress
  • [x] Thumbnail
• [ ] Move Renderer::Parameters to its own file
  • [ ] Add sensible defaults
  • [ ] Add builder pattern for options
  • [ ] Remove separateTracks
  • [ ] Take a vector of Tracks rather than bits
• [ ] Make static functions to create common Render::Parameters
• [ ] Ideally rendered files will be scoped so completion has to take ownership of them or they will be deleted
• [ ] Options to add upstream/downstream tracks
• [ ] Tests:
  • [ ] Two clips on two audio tracks
  • [ ] Submix/folder: Include sub tracks by default
  • [ ] Clip on audio track inside submix
    • [ ] Render audio track
      • [ ] Include upstream submix parent (should be audible with gain change)
      • [ ] Exclude upstream submix parent (should be as if it wasn't in a submix i.e. no gain change)
    • [ ] Render submix
      • [ ] Include downstream (clips should be audible)
    • [ ] Tracks that feed in to other tracks
      • [ ] Two audio tracks, one with a clip which feeds in to the other
      • [ ] Render the source track, should be as if track was rendered by itself (without feeding in to the other)
      • [ ] Render destination track, should render both
  • [ ] Side chain: Source side chain tracks should be included by default
    • [ ] One track with sin tone and sidechain compressor, second track with kick samples feeding side chain
    • [ ] Rendering just the sin track should give the pumping
    • [ ] Rendering just the second should be the normal kick
    • [ ] Rendering both should give the kick and the pumped sin
  • [ ] Mute/solo/isolate: Rendering should not affect the mute/solo states by default
  • [ ] Aux send/return: Include aux sends by default
    • [ ] Option to disable them
  • [ ] Rack source tracks: Don't include Rack source tracks by default
    • [ ] Option to include other Rack source tracks