Wasm mp4 export

[lucasdinonolte]

Follow up PR of #178, but using a WASM version of h264 over Web Codecs to handle the mp4 export.

What this does

• Adds h264Writer class to mechanic core, following the interface of WebmWriter
  • this uses a WASM build of minih264 and libmp4v2 to handle h264 encoding and mp4 multiplexing
• Adds ZipWriter class to mechanic core, following the interface of WebmWriter
  • this uses jszip to export raw frames of an animation and download them as a zip file
  • this is aimed at power users who have very specific video needs that can't be done in the browser (like ProRes)
• Allows animated design function to specify their videoFormat to set the desired export
  • allowed values mp4, webm, and frames (defaults to mp4, as this probably the most wanted format)

What could be done

• Add video export presets
  • right now it just exposes the options that can be passed to the encoder as part of a design functions settings
  • ... but this could be extended to provide presets like "High Quality MP4", "Small File Size Mp4", etc.
• Discuss if we want the export format as an option on the design function or if we should let the end user choose the export format
  • if we decide on the latter there's probably a worthwhile exercise to see how this could fit in with the showMultipleExports option

Caveats

• This seems to increase the build time of a mechanic project, as the WASM for h264 encoding is included inline in the final build
  • It's lazily loaded, so bundle size is not affected by this
  • But it still needs to flow through webpack, making the build slower
• I'm still not 100% sure this is legal – as h264 is not an open codec, so I'm not sure the WASM implementations used by this are legally implementing h264. I think we should get legal advice from a more qualified person before proceeding with this.

Alternatives

Rely on browser standards as outlined in #178 (but with limited browser support for now)

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