Add constraint to render graph

[rparrett]

Fixes #34

I am not sure about #33.

NodeUi::UiPass's ordering constraints seem nonsensical to me. I am probably not understanding it correctly. But I think that as long as we do your stuff before Node2d:: EndMainPassPostProcessing, then this should also fix #33. I'm having trouble actually reproducing the issue though before or after this changeset though.

Not a rendering expert, so this should be scrutinized.

Following the render graph setup in custom_post_processing, it seems that our render graph nodes are constrained to run after Node2d::EndMainPass, but otherwise free to run whenever. Might be before or after any of:

Node2d::Bloom
Node2d::Tonemapping
Node2d::Fxaa
Node2d::Smaa
Node2d::Upscaling
Node2d::ContrastAdaptiveSharpening
Node2d::EndMainPassPostProcessing

I'm not totally sure this is exactly where the nodes belong, but it seems to fix the issue on my end.

I did the following experiments:

(Node2d::Upscaling, SdfPass, LightMapPass, LightingPass) // Seems to fail 100% of the time
(Node2d::Smaa, SdfPass, LightMapPass, LightingPass, Node2d::Upscaling) // Seems to work all the time

So it seems like this definitely needs to run before Node2d::Upscaling.

[rparrett]

Finished looking into #33 about as much as I want to and updated the description.

[jgayfer]

Awesome work. I agree that this is very likely what's been causing the issue. Without referring back to Node2d, the final graph order was very likely not deterministic.

I can reproduce the bug on main pretty easily, and I didn't see it after running this branch 15-20 times. I'm happy to call this The Fix.

Are you able to update CHANGELOG.md with this fix under the unreleased header, referring to this PR? (I should add this to a PR template).

Something like:

### Fixed

-  Non-deterministic render graph, occasionally causing lights to not render and/or affect elements in unintended order (#37).

[jgayfer]

You know what, all good, I can add that changelog entry!