Factor out glrt library and its preamble to compile stand-alone WebGL code

We should factor out the `glrt` library, unify it with the remaining runtime code currently embedded in the WebGL backend, pair it with the dingus preamble file and the “header.ss” file that is currently not well integrated, and make it possible to “link” compiled WebGL code with this library outside of the dingus. This would let developers compile BraidGL code and use it as part of larger JavaScript projects.

It would also let us set up a testing infrastructure for WebGL code. We could use hedadless mode in Chrome to render test images, for example, or use the [headless-gl](https://github.com/stackgl/headless-gl) library.