Run RenderStartup in/before extract instead of after it.

[andriyDev]

Objective

• Fixes RenderStartup runs after ExtractSchedule #19910.

Solution

• First, allow extraction function to be FnMut instead of Fn. FnMut is a superset of Fn anyway, and we only ever call this function once at a time (we would never call this in parallel for different pairs of worlds or something).
• Run the RenderStartup schedule in the extract function with a flag to only do it once.
• Remove all the MainRender stuff.

One sad part here is that now the RenderStartup blocks extraction. So for pipelined rendering, our simulation will be blocked on the first frame while we set up all the rendering resources. I don't see this as a big loss though since A) that is fundamentally what we want here - extraction has to run after RenderStartup, and the only way to do better is to somehow run RenderStartup in parallel with the first simulation frame, and B) without RenderStartup the entire app was blocked on initializing render resources during Plugin construction - so we're not really losing anything here.

Testing

• I ran the custom_post_processing example (which was ported to use RenderStartup in Use RenderStartup in custom_post_processing example #19886) and it still works.

[IceSentry]

I'm okay with blocking extraction on that part personally. We can't really render anything anyway until those resources are initialized.

I'd prefer if the ecs folks reviewed this change though because I have no idea if it's correct to use FnMut here or if this approach fits in the context of everything else.

[kristoff3r]

Looks like a good solution to me, you kinda have to block on the first frame and FnMut is hardly a restriction in this context.

[alice-i-cecile]

One nit, but fundamentally this strategy is totally fine from an ECS perspective. Mutating state in the ExtractFn doesn't cause any problems.

That said, the migration guide is missing advice on what happened to MainRender.

[andriyDev]

@alice-i-cecile MainRender was introduced in #19841 which happened after 0.16, so there's no need for a migration guide for this.

We also didn't add a migration guide for #19841, but I will write one as well as a release note before 0.17 (in a separate PR). I wanted to make more progress on these ports before I write it up.