0.18.0 release

[r-veenstra]

• Breaking change: removed deprecated properties projectDefaultIonAccessToken and projectDefaultIonAccessToken from CesiumDataPrim. CesiumIonServerPrim should be used instead.
• Improved tile streaming performance by 35% by switching to UrlAssetAccessor from vsgCs.
• Added support for disk tile caching which improves streaming performance by 50% when reloading the same scene.
• Added support for Web Map Service (WMS) raster overlays.
• Added support for Tile Map Service (TMS) raster overlays.
• Added support for Web Map Tile Service (WMTS) raster overlays.
• Added raster overlay options: maximumScreenSpaceError, maximumTextureSize, maximumSimultaneousTileLoads, subTileCacheBytes.
• Added ability to bypass downloading of tiles clipped by a cartographic polygon raster overlay.
• Added support for globe anchors on non-georeferenced tilesets.
• Fixed crash when disabling and re-enabling the extension.
• Fixed crash when setting certain /Cesium debug options at runtime.
• Fixed crash when updating tilesets shader inputs.
• Fixed crash when removing USD prims in certain order.
• Fixed issue where Cesium ion session would not resume on reload.
• Fixed issue where save stage dialog would appear when reloading Fabric stage at startup.
• Fixed issue where zooming to tileset extents would not work correctly with non-identity transformation.
• Fixed issue where globe anchors didn't work with xformOp:orient.
• The movie capture tool now waits for tilesets to complete loading before it captures a frame.

[corybarr]

LGTM