Specifically multiply with the lightness column to apply the global dimmer.
Add multiple visual layers that are blended together to get the final output.

* Add a Layer class that manages its own playlist, transitions, and drawing.
* Support blending multiple layers together at the mixer level.
* Remove the defalt playlist from Mixer and FireMixApp.
* Remove transition handling from the Mixer.
* Update GUI to view / make changes to the default layer's playlist.
* Update CombinePresets to take a layer parameter. (super hack, needs
  improvement)
* Add a simple blend_to_buffer method to colors.py to support different layer
  blending operations.