Global 3D Tilesets do not end up with an ideal default camera view

[mramato]

As this sandcastle shows, loading large 3D Tiles tilesets and then calling viewer.zoomTo or viewer.flyTo do not end up with ideal views.

I see two options:

• We could just have it fly to a standard global view if the bounding sphere is some percentage compared to the size of the earth.

• We could leverage an extras property to provide a "default view" for the tilesets, but this requires an extra step.

The second one doesn't really negate the need for the first one, so maybe we do both?

@lilleyse thoughts?

CC @OmarShehata

[lilleyse]

Confirmed that removing https://github.com/CesiumGS/cesium/blob/56866ed98893023f64cb5c3bb2654f5607e5c31a/Source/Widgets/Viewer/Viewer.js#L2115-L2121 doesn't help enough for this case. Both those options sound good.