Imagery feature picking only works in 3D

[pjcozzi]

http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Web%20Map%20Service%20(WMS).html&label=Beginner

[tlpinney]

This occurs because the pickedPosition is coming back undefined in 2D mode.

https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/ImageryLayerCollection.js#L365-L370

It may also be related to the issue #2480

[mramato]

This recently came up on the forum

[kring]

I'm happy to look at this if it's specific to imagery layers, but it seems pretty clear it's a general globe.pick doesn't work in 2D, as described in #2480, right?

[hpinkos]

@kring yeah, I was looking at this a tiny bit earlier today, and globe.pick is definitely the problem. It returns undefined.

[mramato]

Closing since this is a dupe of #2480