Defined g-buffer index constants to make it easier for users to add their own g-buffers

[RoyTheunissen]

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Checklist for PR maker

• Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the PR, the label changes to backported-*. (I don't think this is necessary. It's a cleanup, not a fix)
• Have you updated the changelog? Each package has a CHANGELOG.md file. (Is this worth mentioning in the changelog? It's not a bug fix nor a new feature. I'll add it if you think I should.)
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR. If you do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not sure which person to add, see the Docs team contacts sheet. (I've added comments where relevant, which should be satisfactory)
• Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests don't cover, please add a graphic test. (I don't know what this is exactly or whether it's needed)

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Purpose of this PR

Why is this PR needed, what hard problem is it solving/fixing?

I'm a user and I tried adding a custom g-buffer to URP and it was a real pain 😂 I did a few small changes to make it easier to add a gbuffer and have the other g-buffer indices shift over correctly. I also added some comments to clarify things that I didn't understand, and I'm looking to share those for other users who wish to add g-buffers.

This is one half of changes that I made to my local version of URP to make it easier to expand SurfaceData. If this gets in, I would also like to open a PR with some small changes in shader code, SurfaceData is initialized weirdly in a few places and that cost me a lot of hours to debug, and I think that can be solved elegantly, but this is my first contribution so let's take it one step at a time.

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Testing status

Describe what manual/automated tests were performed for this PR

I have triple-checked that all the values are the same as before, and I've created a test scene with both static & dynamic objects, baked lighting, and then turned a bunch of render pipeline features on and off and checked that all the g-buffers kept working correctly.

That's about as much as I could think of, but if you have any suggestions for other things to test I will happily take a look.

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Comments to reviewers

It's my first contribution. I read the README and contribution guidelines but I was a bit confused about where to branch from & where to merge to exactly, so I set it to Draft so we can maybe check that first.

Thank you for your consideration 🙏

By the way I talked to @UnityAljosha during the Office Hours event on Discord and he gave me permission to open a PR for this.