Implement the layer system

[AMNatty]

• [PlutoRuntime,PlutoCore] Initial implementation of the layer system (formerly known as "stage")
  • A "layer", in this context, is a set of assets bound together
    by programming logic.
    Layer switching and asset management are handled by the engine.
  • Layers can be stacked on top of each other and run sequentially
    from bottom to top
  • Upon layer switch
    1. Unload unused assets
    2. Load new assets
  • Provide multiple means of layer switching
    • Two modes with the initial release, asynchronous switching will come at a later date
      1. Instant switch
         • Assets will be loaded and unloaded synchronously
         • The layer switch will happen in one frame
      2. Deferred switch
         • The layer will continue running until all assets load
         • Assets will load synchronously, but at a slower pace
           to avoid frame stutter
  • Automated asset loading
    • All asset management will eventually be handled by PlutoCore
      • This includes audio clips, textures, sprites
    • Add a common interface for all assets
  • Let the stage system handle audio playback
  • This API should be as neutral as possible and avoid steering towards
    game-only use
  • The stage manager should be relatively low-overhead and allow multiple
    instances
  • Allow stages to be inherited from, creating a stack-like structure
• [PlutoAudio] Integrate the Audio API with the Stage API