Add ST_ClosestPoint

This was previously called
```mysql
ST_NearestPoints(Geometry geomA, Geometry geomB)
```
in GDMS. Two problems with this:

1. The name `ST_NearestPoint` would have been more appropriate, since
   the function returns only one point.
2. The order of the parameters was opposite that of PostGIS.

Now
```mysql
ST_ClosestPoint(Geometry geomA, Geometry geomB)
```
returns the 2D point on `geomA` closest to geomB.

*Note*: If the closest point is not unique (it is easy to imagine an
example where there are infinitely many), `ST_ClosestPoint` returns the
first one it finds. This means that the point returned depends on the
order of the geometry's coordinates.

TODO?: Add ST_3DClosestPoint?