Atmosphere disappears as you look up

[hpinkos]

Reported on the forum https://groups.google.com/forum/?hl=en#!topic/cesium-dev/su5kb_cNN7Y

This seems to have been introduced in 1.16.

• Zoom into the globe and tilt
• Middle mouse drag on the sky to keep tilting up
• The atmosphere flickers then disappears completely when the globe leaves the bottom of the screen

[pjcozzi]

This may have changed in 1.16, but I have reproduced this in older versions.

[lilleyse]

This is still an issue as of 1.21. @likangning93 would you like to take a look at this? You can use the 3D Models Sandcastle demo as an example of this.