Adapted to Magnum changes.
prototypes: oops, both engine forces were applied to left engine.
prototypes: run only one engine at a time.

Previous fix made the engines too powerful, trying to restore the old
behavior. Now only one engine pushes the vehicle, the other (which was
pulling before) is off.
prototypes: removed unneeded "total force" visualization.

Only engine force is applied, so having it visualized twice is useless.
prototypes: better explicit conversion from Complex to Vector2.
prototypes: impulse-based friction.

Does not work properly in large speeds, because spring forces cause near
zero normal velocity and thus friction is 0 in that case. Need to work
around that later.
prototypes: no need to have two tubes for collision.
prototypes: use floats properly.
prototypes: parameter tweaks.

Need to have engine, spring and friction forces in equlibrium so the
vehicle comes to a stop after some time and doesn't escape from the
tube.