Set camera.up vector as the orbit axis in OrbitControls

[WestLangley]

Fixes #4375. Previously, the orbit axis was hardwired to be the y-axis.

We'll see how this is received. Optionally, it could be limited to just z-is-up and y-is-up, which I believe are the two cases of interest. By limiting the options, it would likely be easier to avoid edge cases, which may come up.

Also added a new function that was required for this feature: Quaternion.setFromUnitVectors( vFrom, vTo ), which sets the quaternion to the rotation required to rotate direction vector vFrom to direction vector vTo.

Finally, although unrelated, TrackballControls was generating too many change events. Fixed.

ping @jasongrout
ping @ekatzenstein

[mrdoob]

Sweet! Thanks!

[jasongrout]

Awesome! Thanks! Our 3d graphics in the sage cell server (like http://sagecell.sagemath.org/?q=yrpjbv) will appreciate this!

[jasongrout]

To clarify, if we update the object's up vector after creating the controls, this will not automatically update the controls, right?

[WestLangley]

To clarify, if we update the object's up vector after creating the controls, this will not automatically update the controls, right?

You cannot change the up vector after creating the controls.

This is tricky, and as implemented there are difficult edge cases. That is why I suggested perhaps limiting it to only two options -- y-up and z-up.

[jasongrout]

Sure. My use-case is the z-up, and this seems to work great. Thanks again!