[Feature Request] LookAt function

[Jiayuan-Gu]

How about adding a LookAt function?

[eliphatfs]

Transform-related stuff by design shall go into something like transform3d and that's why I did not include transform hierarchies, compositions or the tricky quaternion/rotation matrix/euler angles in the first place. But maybe we can have a camera helper given in a single convention for the sake of convenience.