I was hoping that I could disable the TrackballControls so that I could
right click and "Save Image", but this event handler was missing the
enabled check.

When using a mouse wheel to zoom in and out quickly, sometimes zoomCamera's factor can go negative. When that happens, _zoomStart.y never gets reset to 0, causing zooming to stop. The fix is to always update _zoomStart, but only update _eye when (factor != 1.0 && zoom > 0.0) .

* Added 'wheel' event as mousewheel has been deprecated in chrome.

When using with Mac and OSX trackpad, orbit controls no longer works in chrome, but safari is fine. Traced it to orbit controls not registered the 'wheel' event.

* Add event listener for 'wheel' due to change in Chrome.

This reverts commit 348c706a.

The `_touchZoomDistanceStart` and `_touchZoomDistanceEnd` resets are not needed since the values will be set in the `touchstart` function when two or more touches are present.

* Removed unnecessary `default` case for two or more touches on `touchend` to prevent issues with 3 or more touches.
* Reset zoom distances on `case 0` and `case 1`. I couldn't combine them easily without doing a separate `if` statement.
* Simplified `case 1` action to set `_moveCurr` and `_movePrev` to the current touch position preventing camera jumping next time `touchmove` is called.

Related to issue #7185 and pull request #7406

Fix for #7185. After using two fingers, the camera would rapidly rotate if both fingers were not removed simultaneously. Adding in some state checks in `touchmove` and `touchend` prevents the jump from the null `_movePrev`, allowing users to quickly go from zooming and panning to rotating.

Fixes #5570

Removed unnecessary `new` Vector in `getMouseProjectionOnBall`.

Removed assignment of variables have already been set.

Made the optional target in `getMouseOnScreen` mandatory.

When using getBoundingClientRect, the returned coordinates are relative to the viewport.  If the page is then scrolled and we have a dom element that is not the document, the top and left offsets are out of date, thus the rotation calculations are off (sometimes by a lot).

In this fix, we instead do all calculations relative to the document (not the viewport) by adjusting the screen.left and screen.top variables with the scroll offset and using pageX/pageY instead of clientX/clientY for the click coordinates.

The start and end events can be used to start or stop an animation loop, for example, so that the animation loop does not have to run continuously.

I haven't tested the problem but it was a bad idea anyways to have start and end have the same referenceObject