Version of quat_interpolate() allowing control over the rotation.

The same as quat_interpolate(), but allows more control over how the
rotation is done. The how parameter can be any one of the values:

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