Fluid collision layers

[lexuv2]

Hey there,

First off, huge thanks for all the hard work on this amazing physics engine! It's been a blast to work with, and I appreciate all the effort that's gone into it.

I've been tinkering around with fluids in my project, and I it would spare me a lot of headaches if I could set collision layers for fluids. I hope this makes sense and isn't too much trouble to consider. Thanks again for everything you do!

[Ughuuu]

Added this to salva already (for some time), needs first to be implemented there: dimforge/salva#57