Automated testing of WebGL rendering

[tuner]

Currently, rendering has no automated testing of any kind. In fact, there are also no proper integration tests! Something could be built with puppeteer and by implementing #75 or just taking a screenshot of the canvas.

• Render the specs in the examples folders as "Golden images", which can be used for comparisona. As some of them use random function, we need a possibility to provide a constant random seed for deterministic results. pixelmatch could be useful here.
• Use vitest or just plain nodejs to write tests that regenerate the images and compare them to the golden images