ZVISION: Remove unnecessary math
ZVISION: Use Common::Rational to simplify fixed point math
ZVISION: Clamp the rotation velocity to never be zero

Before, if we set the in-game preferences to have very low rotation speed,
the velocity ends up always being 0 - 0.99
Hence, when we convert back to an int, everything gets truncated to zero.
Therefore, we clamp, in order to ensure the user can always move, no matter
which setting they use.