Add GEOSsegmentIntersection

[dbaston]

I'm trying to port some code that uses the C++ API to the CAPI.

One portion of the code is using the LineSegment::intersection method to check the intersection of two line segments. This method has no equivalent in the CAPI, and the workaround (creating LineStrings and using GEOSIntersection) increases total program runtime by 15x, even though these tests are a very minor portion of the code.

This ticket is to introduce a function

GEOSSegmentIntersection(ax0, ay0, ax1, ay1, bx0, by0, bx1, by1, rx, ry)

This is clearly a large signature, but without introducing new primitives (Coordinate, LineSegment) to the CAPI it seems unavoidable.

[dbaston]

Added in 9c1c5e0/git