obi fluid + hdrp

[josemendez]

At this point even some simple HDRP shader that is visible would be better than nothing. Obi particle behavior is better than all other fluid systems currently available. It doesn't need to be perfect, but at it's current state nothing can be seen in HDRP which doesn't even allow a use case.

It's being worked on. The reason it hasn't been released yet is that the entire render workflow of all Obi assets is being rewritten for Obi 7. Instead of doubling the amount of work (make a HDRP renderer for the current framework, then immediately throw it in the bin and start re-doing it from scratch), we're writing it directly for the new one.

Some background info on why things are being re-done: Obi 7 includes a compute-shader based physics backend (which for Obi fluid, allows to simulate +60000 particles in under 4 ms/frame on a modern GPU). The thing about GPU physics is that passing any data back to the CPU stalls it, this has such a negative impact on performance that it often negates any benefits of using the GPU at all.

To render stuff using the current framework, we need to have the particle data on the CPU. The only way to do this when using the GPU to simulate physics is to bring back data asynchronously, but this means you trade speed for latency: the data you get is several frames old, which is not acceptable (don't want to render what happened 5 frames ago, but what happened in the current frame!).

So all data must remain on the GPU. This means meshes must be built directly on the GPU without particle data ever setting foot on the CPU. And this of course means we have to ditch the current rendering framework and rewrite all rendering components in Obi.

Good news:
- this benefits all assets and all backends, both from a workflow and a performance standpoint.
- I've been at it for several months now, so it's in a very advanced state. Videos soon.