Add ColorByVelocityModifier

[rydrman]

This modifier sets the color of a particle on a gradient based on its linear velocity at any point in time. Great for having particles disappear as they slow down, for example.

The implementation is largely a copy from the ColorOverLifetimeModifier but calculates the position in the gradient based on a clamped particle velocity

[djeedai]

Hi @rydrman,

Thanks for this change. I had a look, and I think you can achieve this with the current API, by using expressions, can't you? I see we're missing clamp() but I can quickly add that. I'd prefer not to have to support a new modifier if it can be built by users already; the entire point of the Expression API is to have a customizable system so that we don't need built-in features (aside from the intrinsics of WGSL of course). I appreciate that in some cases a helper is useful for common use cases, to help users where for example the math is not trivial. But in this case I don't feel it's clearing that bar. Let me know if you feel otherwise @rydrman (and sorry for letting that PR dangle for so long, I should have replied as such much earlier). Thanks!

[rydrman]

Hmmm, ya I looked at the expression API and couldn't see a path to what I wanted. That being said, I had limited time to get something going. If you feel like it's possible with the expressions then definitely feel free to close - I appreciate you laying out the intention wrt/ the api.