Smooth transition from 3d to 2d

[danny-wang]

If cesium currently has pitch and roll, when switching from 3d to 2d, there will be a problem that the relative positions cannot be completely consistent. I hope that cesium will first transform the viewing angle to be vertically downward in 2 or 3 second, and then switch to 2d. How to achieve this? if viewing is vertically downward, then switch to 2d, view will be completely consistent