Support multiple physics worlds

[tbillington]

It would be useful to be able to work with multiple physics worlds/contexts/scenes at once.

Common uses

• Splitscreen
• Game servers running multiple instances
• Background simulation of separate or "off screen" areas
• Clean separation between different contexts which are being simulated at the same time

This is separate to physics layers.

Prior art

Godot supports multiple PhysicsDirectSpaceState3D's. By default all bodies and areas are assigned to the default world space, however the user can create new spaces and assign bodies/areas to them. Alternatively, setting own_world_3d to true on a Viewport will cause all nodes under it in the hierarchy to exist in a separate physics space.

Unity ECS multiple physics worlds work somewhat similarly. The world an entity is part of is defined by a component storing the index of that world, 0 being the default.

Jolt's support of multiple physics systems.