Change File Format

[cooper667]

We currently use CreateImageBitmap a lot and convert ImageData objects, but just end up storing Base64 Pngs (as the imagedata object isn't serializable). We can likely do something a lot smarter and remove the majority of the conversions (they are probably a lot more 'expensive' than any saving we get from directly rendering an ImageData obj)

It might be worth looking at how Percy serializes canvas data, although they are likely less concerned about speed and size. Must be nice.