Use RenderStartup for SSAO systems.

[andriyDev]

Objective

• Progress towards Use RenderStartup instead of a FromWorld impl whenever possible #19887.
• Develop a utility/common pattern for having conditional systems for RenderStartup.

Solution

• Convert FromWorld impls to systems run in RenderStartup.
• Create conditional_render_set to generalize the pattern of creating system sets + resources to conditionally run systems!

Thanks to @JMS55 for suggesting this generalization in #19918!

Testing

• Ran the SSAO example and it still works!

[andriyDev]

I just noticed that there's quite a bad limitation to the conditional_render_set - there's no way to make sure the checking system runs after another. So if for example your condition depends on GpuPreprocessingSupport, you're kinda out of luck. I'm not sure how to fix this. We could create a generic system set like ConditionalRenderCheckSet that only applies to that checking system. Maybe with a bikeshed we could name that something more reasonable? I'm not sure.

[andriyDev]

I added a little hack for this. If you want to order the condition, you can configure ConditionalRenderSetCondition (the system set) to run after its dependencies.

[atlv24]

makes sense to me, i think the conditional is still nice

[alice-i-cecile]

Hmm. This uses run conditions (not 100% ideal, conditionally adding systems / removing them would be slightly lower overhead), and there's gnarly "configure this system set across multiple schedules" nonsense, which is something that shouldn't be necessary.

That said, there's a lot to be said for having a consistent way to do this, and the exact details are clear and well documented.

Let me think about better patterns, to see if we can get something better now without waiting on further improvements to scheduling.

[alice-i-cecile]

I've thought harder about this, and I really don't like the complexity and ad-hoc nature of conditional_render_set. If we had commands.add_systems, would that work fine for you? That's one straightforward PR away. It's already possible by mutating Schedules, but a command would make it a bit clearer and less boilerplate.

The core request here is "conditionally add systems" and with the RenderStartup changes, we need to move this from plugin-init time to normal systems.

[andriyDev]

@alice-i-cecile I have a couple concerns with that.

First, it means we can't conditionally have systems in RenderStartup, because of course we can't add systems to the schedule we are currently in. That can be worked around by either making a mega system to build all the resources in one system (though some of these systems get pretty big already with a single resource), or you can check whether the feature is supported in the system.

Second, these systems won't show up on tools like bevy_mod_debugdump. It's a little concerning to me to have first-party "sneaky systems". Perhaps in the future, we'll have tools to debug schedules after launch, but that's probably not gonna be the first iteration anyway even for the bevy editor.

These are problems we could live with, but I wasn't excited about these caveats

[alice-i-cecile]

Okay, hmm. I see your point there with the drawbacks of conditionally adding systems. If we're not going to conditionally add systems, a run condition is about as good as it can get.

This is well-encapsulated, so I can swallow it for now. Let me get feedback from the Rendering SMEs before taking on this tech debt though.