Scene edge that has one neighbor, i.e. border edge. It's used by TCastleSceneCore.BorderEdges, and this is crucial for rendering silhouette shadow volumes in OpenGL.
Index to get vertex of this edge. The actual edge's vertexes are not recorded here (this would prevent using TCastleSceneCore.ShareManifoldAndBorderEdges with various scenes from the same animation). You should get them as the VertexIndex and (VertexIndex+1) mod 3 vertexes of the triangle TriangleIndex.