Add user supplied mesh tag

[tychedelia]

Objective

Because of mesh preprocessing, users cannot rely on @builtin(instance_index) in order to reference external data, as the instance index is not stable, either from frame to frame or relative to the total spawn order of mesh instances.

Solution

Add a user supplied mesh index that can be used for referencing external data when drawing instanced meshes.

Closes #13373

Testing

Benchmarked many_cubes showing no difference in total frame time.

Showcase

Screen.Recording.2025-02-02.at.12.29.35.PM.mov

[pcwalton]

In the decal code I have a similar feature, but I called it the "tag". I think that might be a less confusing name than "instance index", because with your code as it currently stands "instance index" has two separate meanings.

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[IceSentry]

I have no idea if the slightly higher memory usage is something we want to care about or not, but other than that this is a really useful PR and the implementation is nice and simple.

[IceSentry]

Yes, I agree that the name is a bit confusing though. I understand why it's named like that and it is factually correct, but potentially confusing.

[tychedelia]

Okay, I think "tag" is a nice alternative for sure especially if it's already used in the engine, I'll switch to that instead 👍 .

[JMS55]

I somewhat wish we could instead always have a tag corresponding to an Entity, and have the instance_index builtin be used to index into a list of tags. So you do draw instance_index within a phase -> tags[instance_index] -> transforms/other_data[tag].

I think we've discussed this on discord in the past, don't remember the conclusion we came to.

[tychedelia]

I somewhat wish we could instead always have a tag corresponding to an Entity, and have the instance_index builtin be used to index into a list of tags. So you do draw instance_index within a phase -> tags[instance_index] -> transforms/other_data[tag].

We just discussed this on discord. This definitely makes sense as a design and could help unify some of these patterns. @IceSentry brought up shader mesh picking as an example of something that could be useful for the engine to ship. I'd be happy if someone implemented that but it will require deeper incisions into things and make my primary use case more complicated. This implementation is still very general and can be used to implement other similar things (e.g. write the entity index into the tag) as well. I do think storing the entity on the gpu may be somewhat inevitable but I'd like to unblock this first, especially to see what other user needs arise.

[IceSentry]

I just did a quick sanity check and it is indeed still a single draw call 🎉