spec_v2: Auto Exposure

[ecoskey]

Objective

#17373
#19969

• Migrate auto_exposure to spec_v2

Solution

• Did the things

Testing

• ran auto_exposure

[andriyDev]

@ecoskey, #20002 changed auto exposure to use systems instead of FromWorld. Can you rebase this PR on that?

P.S., if you're doing more of these migrations, could you make sure you're using RenderStartup instead of FromWorld + initializing in finish?

[ecoskey]

P.S., if you're doing more of these migrations, could you make sure you're using RenderStartup instead of FromWorld + initializing in finish?

Will do, sorry bout that! I'd probably rather do migrate RenderStartup -> migrate spec_v2 in separate PRs so the diffs are smaller, what's left on the list for RenderStartup?

[andriyDev]

No problem at all haha, this is gonna be a battle keeping devs from using finish and instead forcing them to use RenderStartup...

There are 2 cases of finish left in bevy_core_pipeline, a bunch of cases in bevy_pbr, a bunch of cases in bevy_render, some in bevy_solari, and one in bevy_gltf. I don't know how many of these are pipelines we need to convert to spec v2, I still need to work through them. The main blocker is agreeing on a pattern for conditional systems: previously, we would conditionally add systems based on whether some features were supported. Since we are now doing stuff in RenderStartup we can't do this conditional addition of systems, so we need to conditionally run systems, or something like that (proposed in #19918).

Hopefully RenderStartup should be mostly out of your way for spec v2 migrations.