Fix rigid_body_writeback performance scaling

[Shfty]

Currently, the RigidBodyWritebackComponents query used by the rigid_body_writeback system is composed of an Entity and several Option<T> components, with no concrete by-ref / by-mut borrow to prevent it from checking every entity in the world.

This causes it to scale poorly in scenes with many entities. Consider an example scene with two RigidBody cubes and 8000 non-physics background meshes: writeback_rigid_bodies will check all 8002 objects despite only two of them being relevant, which reduces overall frame rate by ~60FPS in my local testing.

This change adds RapierRigidBodyHandle to the query as a concrete by-ref borrow, and passes its inner handle directly to the rest of the system rather than looking it up via entity2body. This prevents irrelevant entities from being queried, and fixes the aforementioned performance deficit.

[Aceeri]

This should be fixed by #361 now

[Vrixyz]

Reminder to use hide whitespace option, this change is very interesting ; I think this should get merged !