BillboardVisualizer.js: dynamic entity position becomes tied to static entity position

[smlawrence]

Reproduced: Sandcastle

https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/DataSources/BillboardVisualizer.js

most recent commit: 337ad9d

line 47: var position = new Cartesian3();
...
line 115: position = Property.getValueOrUndefined(entity._position, time, position);

In the case of a CallbackProperty if the author of the callback returns a variable of non-local scope (to their function) then through subsequent loops of this update method the position of other billboard entities gets written back to that variable via this global variable.

In the Sandcastle example the dynamic entity is under the mouse as it moves and then gets set back to the position of the static entity on each render tick.

Suggestion, to clone it if it exists, otherwise preserve current behavior:

var tempPos = Property.getValueOrUndefined(entity._position, time, position);
if(tempPos) {
 Cesium.Cartesian3.clone(tempPos, position);
} else {
position = tempPos;
}

[hpinkos]

Thanks for the detailed bug report @smlawrence! This made it really easy to track down the problem.
I've submitted a bug fix here: #6815

[smlawrence]

Thank you for looking into and working on that so quickly 👍