Move away from OffscreenCanvas

[TimPietrusky]

• This will allow images (SVG) and videos to be used much easier
• Grandiose will "just" work

Concerns

• Some stuff will break
• Gallery Preview could be limited to improve performance

Ideas

• Put Audio into render-thread
• Output Preview video.requestPictureInPicture
  • Or https://stackoverflow.com/questions/42218566/copy-canvas-image-from-a-browserwindow-to-another-at-60fps-in-atom-electron
  • https://codesandbox.io/s/canvas-pip-qrqdx?file=/src/index.js:392-405
  • https://developer.mozilla.org/en-US/docs/Web/API/MediaStreamTrack/stop

[2xAA]

Closing this as I don't really think it's needed anymore, especially with the addition of the MediaCodec API (#696, #695).
SVG can be handled easily enough by rendering to a Canvas in the DOM and transferring to the worker.