[buffer] fix order when multiple turns are colocated on the same point.
This replaces get_relative_order which was not effectively used anymore and
(most probably) get_left_turns.
This fixes case rt_u7
[buffer][test] add testcase rt_u8, currently still failing
[buffer] remove situation_map because for these sides, rescaling does not give added value and
can cause rounding errors
[test] comment update
[test] enable rt_u8 case and change comments
Merge branch 'develop' into feature/buffer
[overlay] change comment
Merge branch 'develop' of github.com:boostorg/geometry into develop