Add Offscreen.prepare for constructor side effects

[toasted-nutbread]

This is more consistent with other classes. This is probably another largely undocumented style of the project, but in most situations, constructor side effects are avoided and placed into a prepare function. prepare is also a place to put async code which doesn't work in a constructor, but that doesn't directly apply here right now.

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[djahandarie]

LGTM. Should we also turn on the https://eslint.org/docs/latest/rules/no-new lint? It would have (sort of) caught this. Ideally we'd have a stronger lint which checks for global variable access within constructors too, but I guess that doesn't exist.

[toasted-nutbread]

I'll turn it on since in general it's consistent with the style of the project thus far.

Ideally we'd have a stronger lint which checks for global variable access within constructors too, but I guess that doesn't exist.

document and window would run into issues with this, so it's not necessarily the use of globals itself that's a problem.

I've had some thoughts on things related to this before, basically disallowing all global access throughout the project with limited exceptions, including environment globals like chrome, window, document, etc. and instead passing them around more explicitly. This is potentially overkill in some ways, but I could also see it being interesting for some other reasons. I don't really plan on doing this for the time being though.

It's also worth noting that the no-constructor-side-effects (or at least minimal effects) that this project style aims for is not even something that is mandatory when designing software, just something that was chosen. I've seen other JS libraries have plenty of constructor side effects, and it can sometimes simplify API usage overall. So my change here was mainly motivated by consistency.