[buffer][fix] Mark points in buffered areas as not startable instead of discarding them

Within can have in rare cases a rounding issue. We don't discard this
point, so it can be used to continue started rings in traversal. But
will never start a new ring from this type of points.

This fixes p17
Merge branch 'feature/buffer' into develop
Merge branch 'develop' of github.com:boostorg/geometry into develop