[HDRP] Improve decal performances when they use different material and the same draw order.

[alelievr]

Purpose of this PR

Optimize decals for this use case: https://forum.unity.com/threads/decal-projector-performance.965822/

Note that this PR changes the behavior of the order in which decals are rendered when they have the same draw order. This is fine because two decals with the same draw order are not guaranteed to be rendered in a specific order (but the sort is still stable between frames).

---

Testing status

Initial profiling with 500 decals in an empty scene. Decals are all using a unique material spawned with this script:

        for (int i = 0; i < count; i++)
        {
            var d = GameObject.Instantiate(decal, new Vector3(Random.Range(-radius, radius), 0.5f, Random.Range(-radius, radius)), Quaternion.identity, parent.transform);
            var proj = d.GetComponent<DecalProjector>();
            proj.material = new Material(proj.material);
        }

Decal projector settings are identical for every decal:

The decal shader is a ShaderGraph with a simple gradient noise animated based on time.

This is without the fix in CPU profiling:

With the fix:

As you can see on the CPU side there is a 5x improvement. GPU side is identical since the decal list is the same, only order changes.

Notes for the future

We may benefit from a proper instancing system for decals, meaning that our decals sets will be stored per shader and not per material. This would remove the cost of having to go through the initialization of jobs for every decal material in a scene (this is currently the next big bottleneck on decals). Though that would involve a lot of work since all the ShaderGraph / decal properties would need to be instanced in the shader.

Yamato

https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/hd%252Fimprove-decal-material-perfs/.yamato%252Fall-hdrp.yml%2523PR_HDRP_trunk/9969232/job

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
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[kecho]

Good change!

[RemyUnity]

Ship it !

[sebastienlagarde]

Please, add changelog

[sebastienlagarde]

please fix changelog