Add minimal program for Issue #781

[afabri]

Summary of Changes

Added the minimal test case that shows the error reported in Issue #781

Todo: Fix the bug. @lrineau proposed something (see disussion in the other issue)

Release Management

• Affected package(s): Mesh_2
• Issue(s) solved (if any):
• License and copyright ownership: unchanged

[afabri]

The input is a segment and a point close to the segment. So the domain is just a triangle. The first refinement point is the midpoint of an edge, and it is outside of the domain.
The assertion is triggered in Refine_edges_base::conflicts_zone_impl(), namely in Line 429