[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