Feature missing when the clipped geometry is a mixed GeometryCollection. #50
Description
For example, when a polygon's intersection of buffered tile envelope is a GeometryCollection which consists of a Point and a Polygon, this GeometryCollection will be omitted from the encoded bytes of tile.
Because the commands(geometry Geometry) method only process MultiPoint, MultiLineString, MultiPolygon, Point, LineString and Polygon, so we should convert the clipped mixed GeometryCollection to one of them.