Update CesiumWebMapTileServiceRasterOverlay to match Unity implementation

[j9liu]

This was a followup to CesiumGS/cesium-unity#446 that I didn't get a chance to do until now :') From changelog:

• Renamed the following properties on CesiumWebMapTileServiceRasterOverlay:
  • Url -> BaseUrl
  • West -> RectangleWest
  • South -> RectangleSouth
  • East -> RectangleEast
  • North -> RectangleNorth
• UseWebMercatorProjection has been deprecated on CesiumWebMapTileServiceRasterOverlay. Instead, use the Projection enum property to specify between Geographic or WebMercator projection.
• Added TileWidth and TileHeight properties to CesiumWebMapTileServiceRasterOverlay.
• Fixed a bug where CesiumTileMapServiceRasterOverlay, CesiumWebMapServiceRasterOverlay, and CesiumWebMapTileServiceRasterOverlay would attempt to load empty URLs.

You can test that these changes upgrade seamlessly by:

1. BEFORE checking out this branch, add a WMTS overlay to Cesium World Terrain. Put some non-default values in the properties.
2. Check out + build this branch, and reopen the level.
3. Confirm that the property values were preserved.

[csciguy8]

Thanks @j9liu !

Code looks great. Verified that old maps load properly and we don't get console spam with empty urls.

Tested using the "USGS Shaded Relief" WMTS in the "Imagery Layers Manipulation" CesiumJS Sandcastle example.