Add visualization of 3D particle emission shapes

[paddy-exe]

Adds support for visualization of particle emission shapes.

Separated part of #86902 due to discussion with @QbieShay on RocketChat

TODO

• Implement support for ring_cone_angle (with support of @Arnklit)

Showcase

[QbieShay]

Should the signal emission be gated behind a tools build?

[paddy-exe]

Should the signal emission be gated behind a tools build?

Hmm I guess we could put it behind a tools build flag 🤔 not sure if we have to though. Do you think we should?

[Calinou]

Hmm I guess we could put it behind a tools build flag 🤔 not sure if we have to though. Do you think we should?

I'd say yes, if the signal has no purpose outside of the editor. If there's strong demand, the signal could be made available for use in projects later, but that would need a separate proposal to gauge interest first.

[QbieShay]

My rationale here is that I don't want this to cause performance issues in case people are animating the size (for whatever reason).

Alternatively instead of using a signal, it could be polled, but i don't know if gizmos support it.

[paddy-exe]

@QbieShay I think I added this now

[Arnklit]

The above suggestion should correct the ring gizmo. Needs to be added for CPU as well, but it should be identical.

godot.windows.editor.dev.x86_64_1KYwAabcbW.mp4

[paddy-exe]

• Added commit from Kasper
• Added CPU Particle support for ring cone angle

• Cleaned up the code comments

Ready for review now 👍🏻

[Geometror]

Tested the PR locally, everything works except changing the emission shape offset/scale which emits the wrong signal (see comment below).

Small nitpick since this is undefined behavior; when setting the emission ring radius to 0 and the emission ring inner radius to something > 0, one of the rings disappears:

I mean we shouldn't allow this at all (add checks to the setters of ParticleProcessMaterial accordingly), but that's for another PR.

[AThousandShips]

Looks great! Haven't tested but code looks good

[Geometror]

Retested to ensure nothing broke. Great work!
Just a few missing consts left, then I think it's good to go.

[paddy-exe]

Done now and thanks for the detailed feedback 👍🏻

[Calinou]

Tested locally, it works as expected with both GPUParticles3D and CPUParticles3D.

Code and docs look good to me.

While testing, I found an unrelated bug: #100946

[akien-mga]

Thanks!