void quat_slerp(const QUAT *from, const QUAT *to, float t, QUAT *out, int how);
QUAT_SHORT - like quat_interpolate(), use shortest path
QUAT_LONG - rotation will be greater than 180 degrees
QUAT_CW - rotate clockwise when viewed from above
QUAT_CCW - rotate counterclockwise when viewed from above
QUAT_USER - the quaternions are interpolated exactly as
given