Support high-quality raster overlays on additive-refined tilesets

[kring]

Adding a raster overlay to a tileset that uses additive refinement, like Cesium OSM Buildings, doesn't work very well. The problem is that detail added in large, low-LOD tiles (like large buildings in Cesium OSM Buildings) continues to be rendered when zoomed in close (this is what additive refinement means). But a single raster overlay texture cannot have enough detail to cover the large area covered by the tile with sufficient detail to look good up close.

I think the best solution is to switch to "replace" instead of "add" mode when adding a raster overlay, by slicing and duplicating geometry into child tiles.

Reported here: https://community.cesium.com/t/blurry-ion-raster-overlay/26427

[kring]

It's possible some sort of solution based on virtual texturing could work, too, but I don't know the details.