Shift isn't used as that would affect joystick controls and we
probably should discourage the user from roaming around in external
cam mode too much, so they won't forget that they're not in regular
free fly out of the cab.
Update Init(...) parameters to glm::dvec3, change LookAt to glm::dvec3 and adjust Camera.cpp to remove redundant glm::dvec3 casts (lookAt call and vector subtraction). These changes improve precision for camera lookAt vector in world coordinates above 4 digits
