Refactor encoder settings and scene settings api

[chrisgervang]

• scene settings are defined at scene load time, so you can set scene resolution and animation length there.

interface DeckSceneParams {
  lengthMs: number
  width: number
  height: number
  animationLoop: any
  keyframes: any
  data: any
  renderLayers: (scene: DeckScene) => any[]
}

• encoder settings are defined before every render, so you can set seek options there.

interface EncoderSettings {
  startOffsetMs?: number
  durationMs?: number  
  filename: string
  framerate: number
  jpeg: {
    quality: number
  },
  webm: {
    quality: number
  }
}