Remove ambient light as a resource

[alice-i-cecile]

I'd be in favor of marking the resource as deprecated, but we can do that later.

Originally posted by @pcwalton in #17343 (review)

I agree with this: there should be one clear way to do this.

[alice-i-cecile]

I don't think we can mark the Resource implementation as deprecated due to limitations in Rust. See rust-lang/rust#39935.

[IceSentry]

I'm not sure about that. It's used as a way to configure a global default now. Which I think makes sense to keep?

Like, if someone wants no ambient light and we remove the resource, they would need to manually set it to 0 on all cameras if we remove the resource.

[alice-i-cecile]

Can't we just treat "no value set on camera" as 0?

[mockersf]

no strong opinion for me, but if we remove it as a resource we should probably reverse the requirement and instead of ambient light requiring a camera, having camera require an ambient light

[IceSentry]

Can't we just treat "no value set on camera" as 0?

It isn't just about setting it to 0 though. If for example someone wanted a non-default ambient light applied to every camera and disable it only on one then they would again need to update all their camera.

To be clear, it would probably be fine for most people, but that would be one reason to keep it around.