Invert option for polygon raster overlays

[r-veenstra]

Add support for inverting the polygon raster overlay as per Cesium for Unreal. Make clipping exclude outside of the polygon instead of inside.

cc @corybarr

[corybarr]

Removing the inherited CesiumCartographicPolygon prim makes this challenging. With the prim, we can easily add a corresponding property to the schema, which I believe is the proper way to do this. We can create the invert attribute during instantiation at the Python level. However, this has disadvantages:

• There's no guarantee that the prim will have the attribute. The responsibility is on the developer to add it after the Define.
• It doesn't play nice with UI widgets. The raw name of the attribute will be the display name, and the attribute will only appear in the Raw USD Properties:

[r-veenstra]

@corybarr doesn't this just live on the CesiumPolygonRasterOverlay prim schema, which should still exist? In Unreal its a property of the overlay, as opposed to individual polygons.

[corybarr]

@corybarr doesn't this just live on the CesiumPolygonRasterOverlay prim schema, which should still exist? In Unreal its a property of the overlay, as opposed to individual polygons.

@r-veenstra Yeah, you're right. I just came back here to add that. Seems I worked a bit too much today.