Round bouncy velocities.

In manual testing, I could get entities to get in this weird positive
feedback loop where bounciness was acting like it was greater than 1
when set to 1. Couldn't make a test for it which rankles, but rounding
the velocity made it impossible for me to reproduce where, before, it
would take 1 out of 4 tries to get it to happen.

Probably due to vagaries of float representation and having a fracked
timestep. I'll keep investigating.