Fix OffsetCurve parameter handling

[dr-jts]

Fixes OffsetCurve to ignore the value of BufferParmeters.endCapStyle and always use CAP_ROUND.
Also removes the unnecessary setting of this value in GEOSOffsetCurve_r.

Fixes #897

[strk]

Should it have always used CAP_FLAT instead ?

[strk]

See qgis/QGIS#53165 (comment)

[dr-jts]

Should it have always used CAP_FLAT instead ?

Not according to that unit test, and the way I wrote the algorithm.

[dr-jts]

From the issue in qgis/QGIS#53165 it appears that using Quadrant Segments = 0 (or any small value) causes artifacts to appear. The QuadrantSegments value should be forced to be >= 8 when building offset curves. (Even better would be an ability to specify the End Cap Quadrant Segments independently of the Join QS).

The cause of the problem is that a small QS value can expose "closing arc" linework at internal corners. This is a problem with generating buffers as well.

It also might be possible to detect and remove unwanted end cap segments when the QuadrantSegments value is small (at the ends of the offset curve, via a distance check).

[dr-jts]

See locationtech/jts#980