When in the sky position, the rotation event is invalidated.

Hi,I'm not sure if this is a bug, but sometimes the experience is not very good.
When the earth is rotated to a certain angle of view and a large area of sky appears, if you place the mouse on the sky, you will find that the mouse rotation event does not work, you must put the mouse back on the ground. The second is that after changing the angle of view to a front angle, if you adjust the angle of view to the original oblique angle, you will find that the distance between the object and the camera is farther than before, and you need to keep pulling in the angle of view, that is Even if the others are not changed, the viewing distances of the two times are different, which makes the user feel confused. I hope the operation of cesium can become smoother, thank you.

https://user-images.githubusercontent.com/10000818/102994761-a60a5400-455a-11eb-861e-13e3b9fbc27e.mp4

Start by reproducing the two camera-interaction problems shown in the linked video: rotation failing over the sky and inconsistent viewing distance after changing angles. Trace the Cesium viewer's mouse rotation and camera-distance handling; done means both interactions remain consistent when switching between sky and ground views.

Written by the indexing model from the issue text.