Added screenshake system

Entities can have a Screenshake component which makes them a screenshake
source. This component must contain an intensity field and a falloff
field. Screenshake sources can also be timed by adding a Lifetime
component to the entity.
Misc. nonfunctional changes to game.lua

...including removing some obsolete Force components
Attempted to fix text rendering by flooring coords
Tweaked and cleaned screenshake files
Adjusted test scenario