Custom Fluid-fluid interaction groups

[ThierryBerger]

• Follow up to Add an interaction group to specify fluid/boundary interactions #62

in the following screenshot, blue and green fluid layers are not colliding:

[ThierryBerger]

Tagging @Ughuuu as you expressed interest in this.

[Ughuuu]

Woah! Will try to integrate into godot-rapier sometime also!

[sebcrozet]

Thanks!