Use RenderStartup for bevy_solari.

[andriyDev]

Objective

• Progress towards Use RenderStartup instead of a FromWorld impl whenever possible #19887.

Solution

• Convert any resource mutations into systems (including resource init).
• Add the render graph nodes from systems.
• Create a SolariSystems system set that all systems belong to.
• Check if the required features are present in a system and insert a resource if they are.
• Add a run condition to SolariSystems to only execute systems if the resource is present.

Testing

• Ran the solari example, and it still works.

[andriyDev]

This PR is based off of #19912 so we don't have to restructure how we add render graph nodes. I will wait for that before sending it out for review.

[JMS55]

I really don't like this solution :/. It really complicates the solari code.

Perhaps we could make a generic version of this that multiple plugins could use?

[andriyDev]

@JMS55 I'm not sure I agree that this complicates the solari code significantly. The only part that isn't specific to solari code is I think the fact that we have to configure the system sets 3 times.

I tried drafting this up and it's quite gross 0da4a59

In theory the conditional_render_set function would exist in bevy_render or something. But this leads to a lot of complexity that is just hidden. And I suspect it's going to be more difficult to debug because of these generic system sets.

To me, this seems like a lot of complexity that makes it less clear what's going on and why.

---

Looking through our code, here are these cases with finish impls that check some resources before inserting stuff.

• crates\bevy_core_pipeline\src\oit\resolve\mod.rs
• crates\bevy_pbr\src\atmosphere\mod.rs
• crates\bevy_pbr\src\meshlet\mod.rs
• crates\bevy_pbr\src\render\gpu_preprocess.rs
• crates\bevy_pbr\src\render\mesh.rs
• crates\bevy_pbr\src\ssao\mod.rs

Feel free to investigate, but the TL;DR for what these conditionals access is:

• features from RenderDevice
• limits from RenderDevice
• "downlevel_capabilities" from RenderAdapter
• allowed usages for textures in RenderAdapter
• is_available from GpuPreprocessingSupport.

That's quite a mix of things we access, so it's not as simple as adding a run condition for features or something (plus I want to make the run conditions themselves as cheap as possible).