ST_ClosestPoint: return 3D point if passed in 3D point

[komzpa]

We have lines in X,Y,Z that have Z storing timestamps of traveling through points.

ST_ClosestPoint drops Z instead of interpolating it. ST_3DClosestPoint measures distance also across the fake time which is not what we would need.

ST_ClosestPoint should return 3D points for 3D inputs.

[robe]

I disagree — that should go in ST_3DClosestPoint.

We standardized on the ST_ dealing 2D and the 3D ones for 3D + more