Add FlipCoordinates

[peterstace]

• Add a new method to all geometry types FlipCoordinates() T. Here, T is a pseudo type that's the same as the type the method is defined on. So when implemented on Point, it should be FlipCoordinates() Point.
• It should use TransformXY internally. Note that TransformXY can return an error.
• Because geometries cannot become invalid by flipping their coordinates, we can panic on the error, and disable geometry validation via the ctor option for performance.