Add support for viewing tiles in Iso orientation

[csciguy8]

Open cesium-unity-samples, 02_CesiumMelbourne

Look down at the street somewhere...

Switch the orientation to Iso by using the Scene Gizmo (circled in red in previous screenshot)...

Tiles are missing.

Interestingly, keeping the camera in Persp and setting the FOV to 4 degrees (minimum) looks fine.

• Unity 2022.3.21f1
• Windows 10
• Cesium for Unity 1.8.0

[kring]

Same issue in Cesium for Unreal: CesiumGS/cesium-unreal#383
The SSE-based LOD selection algorithm used by 3D Tiles doesn't work with non-perspective frustums. I'm sure there's something reasonable we could do instead, but it's never been deemed important enough to figure out.

[csciguy8]

One use case for ortho views would be when defining a spline for a Cartographic Polygon.

Using a top down ortho view makes it a little easier to line up a spline to features in the imagery (roads, rivers, other boundaries).

[j9liu]

Here's an old (closed) PR we might be able to use for future reference: CesiumGS/cesium-native#287