Modifying gravity doesn't wake sleeping colliders

[rparrett]

To repro:

• Add a dynamic rigidbody
• Configure rapier so that gravity is disabled (Vec3::ZERO)
• Wait for the rigidbody to go to sleep
• Enable gravity

And observe the rigidbody floating mid-air.

[Vrixyz]

We should consider performance when tackling this, if we have a lot of objects, but change gravity to be only slighty more powerful, chances are we don't want to wake up all our tiny objects (and most likely just not wake up anything at all.

I wouldn't mind a wake_up_all_rigibodies() function, but this needs to be explicit in my opinion.