Expunge fidelity

[elalish]

Thanks to Khronos for taking over maintenance of our render fidelity comparisons! Now we can remove them from this repo, thus reducing our size considerably. This will mean that git clone --depth=1 will be nice and fast now. Of course our history is still bloated with gigabytes of these golden images - I'm tempted to rewrite our history to remove them, but that's slightly more dangerous that I feel comfortable with at the moment.

We're keeping the render-fidelity-tools package because we use it for render regression testing, particularly when we update three.js versions. But we've now removed all the other renderers, which also speeds our build process.

I also did a follow-on to #4543, to speed up render-goldens the same way npm test was speeded up: by keeping the browser page open and rendering each scenario inside.