ShapeKitPolygon now also loads interiors when using initWithGeosGeometry and initWithWKT

Rewrote parts of the code to extract fetching ExteriorRing and Interios from GEOSGeom. Was only used in initWithWKB, now also for initWithGeosGeometry and initWithWKT.

Also extracted some code in ShapeKitPoint to prevent duplicates.

Overall more readable code, and easier to maintain :)