Handle web mercator raster overlays on geometry tiles that extend outside the web mercator bounds

Web Mercator texture coordinates get dodgy north/south of ~+/-85 degrees latitude. CesiumJS deals with this by reprojecting web mercator imagery tiles to geographic when they're attached to terrain tiles that cross this boundary. We could do the same here, or alternatively we could insert extra vertices in the geometry at ~+/-85 degrees latitude so that we never try to texture map across this boundary.