[buffer][fix] Avoid check neighbors on the border for inside, that
should not be the case (but might, by rounding errors)
[buffer][fix] don't check within concave helper pieces
[buffer][fix] Don't add piece if strategy returns false
[buffer][test] enabled field_sprayer test, added aimes-167 test
Both tests are disabled for flat-ends for the same reason: flat ends should
be intersected with helper-segments too
[buffer][test] increased tolerance for aimes test
Most of them now succeed, with exception of 6 cases
[buffer][test] Exclude failing tests (Aimes),
include succeeding test (O1)
avoid reporting (false) self-intersection (aimes120)