ConcaveBoundaryTest unit test

This is from the case I originally got to trigger the problem.  We could
probably boil it down to something smaller (by temporarily reverting the
Orient2d change to make sure the failure still occurs) but this doesn't
seem too bloated for a unit test as-is.

unittest/main.cpp

@@ -67,6 +67,52 @@ BOOST_AUTO_TEST_CASE(NarrowQuadTest)
   }
 }
 
+BOOST_AUTO_TEST_CASE(ConcaveBoundaryTest)
+{
+  // Concave-by-less-than-epsilon boundaries used to potentially fail
+  // during triangulation
+  const double eps = 1e-15; // This gave EdgeEvent - null triangle
+  std::vector<p2t::Point*> polyline {
+    new p2t::Point(0,0),
+    new p2t::Point(0.5,eps),
+    new p2t::Point(1,0),
+    new p2t::Point(1-eps,0.836541),
+    new p2t::Point(1,2),
+    new p2t::Point(.46,1.46+eps),
+    new p2t::Point(0,1),
+    new p2t::Point(eps,0.5)
+  };
+
+  const double r2o4 = std::sqrt(2.)/4;
+  std::vector<p2t::Point*> hole {
+    new p2t::Point(0.5+r2o4,0.5),
+    new p2t::Point(0.5,0.5+r2o4),
+    new p2t::Point(0.5-r2o4,0.5),
+    new p2t::Point(0.5-eps,0.5-r2o4)
+  };
+
+  std::vector<p2t::Point> interior_points
+    {{0.21,0.79},{0.21,0.21},{0.79,0.21}};
+
+  p2t::CDT cdt{ polyline };
+  cdt.AddHole(hole);
+
+  for (auto & p : interior_points)
+    cdt.AddPoint(&p);
+
+  BOOST_CHECK_NO_THROW(cdt.Triangulate());
+  const auto result = cdt.GetTriangles();
+  BOOST_REQUIRE_EQUAL(result.size(), 18);
+  BOOST_CHECK(p2t::IsDelaunay(result));
+  for (const auto p : polyline) {
+    delete p;
+  }
+  for (const auto p : hole) {
+    delete p;
+  }
+}
+
+
 BOOST_AUTO_TEST_CASE(TestbedFilesTest)
 {
   for (const auto& filename : { "custom.dat", "diamond.dat", "star.dat", "test.dat" }) {