planar.PolygonContains doesn't work for Islands from a single large polygon

[acheong08]

For example, using this shapefile, it fails to identify Hainan as part of the shape.

Code used: https://github.com/acheong08/apple-corelocation-experiments/blob/main/cmd/exploration/main.go

https://github.com/acheong08/apple-corelocation-experiments/blob/main/lib/shapefiles/china.go

Failing test: https://github.com/acheong08/apple-corelocation-experiments/blob/f89052a456328011d30a761333f296ad31549c2d/lib/shapefiles/shapefiles_test.go#L54-L58