[buffer][test] change usage of join strategies for robustness check
[buffer] remove unused variable
[buffer] don't check neighbor for on-border
[buffer][test] remove unused variable
[buffer][fix] In rare cases an extra ring was traversed and created. Fixed, that ring is
discarded now. This fixes behaviour of assign_parents with positive-rings in positive-rings (when
check_for_orientation is true; normally (for overlay) it is false)

It also reverses a negative-only ring so fixes a linestring_buffer testcase.

For multi-polygon it fixes new testcase rt_u2 found by recursive-polygon-buffer test
[buffer][test] clean up old comments/cases
[buffer][fix] Fix accidental removal of == 0
[buffer][fix] Fix turns which were only on helper-segments, they can be discarded.
This fixes rt_u3 and rt_u4 where a false interior ring was generated
[buffer][fix] Distinguish between "within" and "within_near_offsetted"
The last one might be offsetted by 1 due to integer rounding. It is not
selectable as a starting point but may be used for traversal
[buffer][fix] use taxicab distance instead of comparable distance
This is faster and avoids overflows. This fixes rt_u6
[buffer][test] improve piece labelling on round joins
[buffer][test] Minor change in comments/whitespace
[buffer][test] Add rt_u7 which is currently failing because of tripled IP,
needing get_left_turn info again. Added debug info
[buffer] removed wrong comments
[buffer] remove taxicab distance, comparable distance is OK
Merge branch 'develop' into feature/buffer
[buffer][fix] fix neighbour check in calculating turns. This fixes ticket 10398
[buffer] remove now redundant member m_last_piece_index
Merge branch 'develop' into feature/buffer
[doc] add fixed ticket to release notes