0 = snap, 1 = no change.
The up direction the new orientation keeps. Defaults to the
camera's own up, which for fly controls is the current up (roll
included): a look-at turns the view without dropping the roll. It was a
hard-coded world +Y, so re-targeting (a restored pose, a framing on a
Z-up scene) silently replaced the authored up.
Smoothly slerp the orientation toward a quaternion that looks at
targetfrom the current camera position, then sync the camera.