Add ST_MakeEllipse

Just one signature is available:
```mysql
SELECT ST_MakeEllipse(ST_MakePoint(2, -3), 6, 4);
```
This creates an ellipse centered at `POINT(2 3)` with width 6 and height 4.

The only difference between a circle constructed using `ST_MakeEllipse`
and a circle contructed using `ST_Buffer` is the number of line segments
in the approximation.  `ST_MakeEllipse` is more fine-grained (100
segments rather than 32).

*Note*: In GDMS, the following signatures were available:
```mysql
SELECT ST_MakeEllipse(ST_MakePoint(2, -3), 6, 4);
SELECT ST_MakeEllipse(2, -3, 6, 4);
SELECT ST_MakeEllipse(2, -3, 6, 4, PI()/3);
```
The rotation part can be taken care of by `ST_Rotate`.