Enable terrain by default in CesiumViewer

[hpinkos]

Fixes #6192

@mramato is there anything else I had to do to enable clamping for the data sources? We were already passing in clampToGround for GeoJsonDataSource and for CzmlDataSource it's something you enable in the CZML, right?

[cesium-concierge]

@hpinkos, thanks for the pull request! Maintainers, we have a signed CLA from @hpinkos, so you can review this at any time.

⚠️ I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

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[pjcozzi]

I think we should update CHANGES.md for this.

[hpinkos]

@pjcozzi done

[mramato]

This causes the BaseLayerPicker to still think Ellipsoid is selected. To fix that, you need to tell it to use terrain as configured by the picker. Because there's a chance we might not have the baseLayer picker (because of the imagery provider options), we need to handle both cases. Basically this code becomes:

    var baseLayerPicker = viewer.baseLayerPicker;
    if (defined(baseLayerPicker)) {
        var viewModel = baseLayerPicker.viewModel;
        viewModel.selectedTerrain = viewModel.terrainProviderViewModels[1];
    } else {
        viewer.terrainProvider = new CesiumTerrainProvider({
            url: 'https://assets.agi.com/stk-terrain/v1/tilesets/world/tiles',
            requestWaterMask: true,
            requestVertexNormals: true
        });
    }

[mramato]

As an aside, we can probably get rid of the imagery provider option, but for this PR I wouldn't worry about it.

[hpinkos]

Thanks @mramato, fixed

[mramato]

Thanks @hpinkos!