Export without PNG

[zaun]

Is your feature request related to a problem? Please describe.
The lpe format is nice, but it still has the base64 png image data. It would be useful to have an array of numbers to the color index so it would be usable in other apps that could use the raw image data rather than PNG.

Describe the solution you'd like
Have an option to export pure JSON without embedded png image.

Additional context
Allows for outside usage of the image data. It's probably a niche case usage but it would save from having to use external tools to convert a PNG into an index array.