include json data in exported png images

[roipoussiere]

The json data could be included in an exported png image (#32) using steganography. This should be eventually disabled using a cli option. The image should contain some visual hints about the fact it actually contains the model, such as This image contain actual model data, view it on https://.... The file extension could be eventually something like .cq.png.

A static website could read those images and display code and 3d model.

This way, CQ snippets could be easily shared, for instance on CQ Discord. Good alternative to #41.