[buffer] extend piece analyses, check helper-segments for on-offsetted, on-original,
disjointness. This removes the workaround w.r.t. has_concave
[buffer][fix] oops, use geometry instead of bg
[buffer] add specific robust bounding box for offsetted part.
This gains in some cases more than 10% performance

This gives two small changes in expected areas for country buffer deflate-5, which are not
visible and not detectable using other tools (spent quite some time to try)