Add Collision Layers and Masks.

[kakoeimon]

Right now all the colliders collide with each other, there is no way to tell which colliders can collide.
One way to do this are collision layers and masks.

A collision layer is where the given collider leaves and the mask is which layers can the collider collide.
Layer and mask are just a 32 bit integer and this makes the comparisson very fast.
Godot Game engine uses this technic
An example of setting individual bits can been seen here :
https://github.com/godotengine/godot/blob/00949f0c5fcc6a4f8382a4a97d5591fd9ec380f8/scene/2d/area_2d.cpp#L483

And the comparison here:
https://github.com/godotengine/godot/blob/8e8699e36b81f04fb7acb403bb567648f576867f/servers/physics_2d/collision_object_2d_sw.h#L193

[sebcrozet]

Thank you for detailing your needs here. This is similar to what exists in ncollide as collision groups. This should be integrated to Rapier.

[stephanemagnenat]

Box2D does it the same way as Godot it seems.

[Gregoor]

I have never implemented anything like this in a physics engine and overall my experience working with real physics engines (that weren't my own hacky beasts) is pretty non-existant. But I'd take a shot at looking at what ncollide is doing and porting it over, if that might be valuable @sebcrozet?