Cesium sync occuring in unsupported projections

[wallw-teal]

We prevent switching into 3D mode when in projections that Cesium does not support (anything outside of EPSG:3857 or EPSG:4326).

1. Add a message for that so that it doesn't just appear that the button is suddenly busted

If you're in 3D, and switch to 2D, then we continue to do the Cesium sync in the background so that flipping back to 3D is seamless.

2. Either stop doing that, or only do it if the application is in a projection that Cesium supports. Flipping over to something like EPSG:3031 produces errors in the Cesium sync.

@justin-bits this is item #1 you mentioned here