Fix buffer to remove inverted ring curves

[dr-jts]

This is a fix for the following buffer situation. It can happen that the sides of a small, convex polygon produce offset segments which all cross one another to form a curve with inverted orientation. This happens at buffer distances slightly greater than the distance at which the buffer should disappear. The inverted curve will produce an incorrect non-empty buffer (for a shell) or an incorrect hole (for a hole). It must be discarded from the set of offset curves used in the buffer.

Heuristics are used to reduce the number of cases which area checked, for efficiency and correctness.

This is a fix for the issue described in #472.

Signed-off-by: Martin Davis [email protected]

[dr-jts]

Fixes #472

See also GEOS 1101 and QGIS 41696.