Fix physics timestep

[bergice]

• Fixes physics ticks per second always running at 20 ticks per second • Uses fixed timestep to avoid unpredictable physics stepping at variable updates

Note:

• Does not implement delta timer for other physics operations (velocity etc), so if creators change the physics rate, they have to change other things like movement speed etc. • Resolves https://trello.com/c/AigAP26I/8792-fix-physics-updates-per-second-game-setting-not-actually-working