game: improve player movement

make player move more realistically:
* disable fixed rotation, player now rotates on
  friction and impacts (like missile hits)
* velocity can now be impacted by missiles, friction
  and restitution

in order to achieve this we had to make a little hack
with linear damping, without this it wouldn't be possible
to keep instantaneous move/rotations, which we want to
keep, at least for now...