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

[drowaudio]

• High level API that takes an Renderer::Parameters and renders an Edit on a background thread
• Returns immediately with a handle
• Runs on a an internal background thread
• Takes a std::function that it calls on completion
• Handle:
  • Progress
  • 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