Add FluidSim class for fluid simulation

[bandinopla]

Made an object to apply fluid simulations based on a WEBGL code written originally by (2017) Pavel Dobryakov : (https://github.com/PavelDoGreat/WebGL-Fluid-Simulation/tree/master)

The object allows to quickly setup a surface that reacts to other "tracked" objects affecting the "fluid" and also it offers otter settings to change it's behaviour. The interface is very simple and allows to also maybe used not just as a fluid but as smoke? or clouds... It has many uses.

DEMO: https://threejs-fluid-simulation.vercel.app/

[Mugen87]

There has already been added a fluid simulation for WebGPURenderer: https://threejs.org/examples/?q=fluid#webgpu_compute_particles_fluid

There are also the following GPU water demons for both WebGLRenderer and WebGPURenderer:

https://threejs.org/examples/webgl_gpgpu_water
https://threejs.org/examples/webgpu_compute_water.html

I would rather suggest to improve one of the existing implementations than adding a third one.

Besides, adding such a component for WebGLRenderer means it relies on a lot of custom GLSL code. Please understand that it is not beneficial for the project to add more of this type of code because it is hard to maintain and must be converted to TSL in the future.

Thanks for your effort here but at the moment I think it's better if FluidSimulator is not added to the main repository but is maintained in an external one.

[bandinopla]

this reminds me of: https://www.youtube.com/watch?v=W7rSYzbpA8k
Also if the word "fluid" makes noise, just call it "smoke simulator" and that argument you just did falls apart.
Anyway, no problem.

[Mugen87]

It's an unrewarding task to tell developers when a PR is rejected but it is indispensable to keep an eye on the maintainability of a project. There is a so many third-party code that could be added to the main repo but at some point the amount of code is so overwhelming that we can't focus on developing the core anymore. So stuff like the renderer or material system.

The addons folder is already quite large so focusing is important. And focusing means from time-to-time saying "no". I'm sorry but I feel the PR is just out-of-scope. Maybe other collaborators evaluate this issue differently but my vote stands.

[bandinopla]

Sweet rejection...