Fix of mesh decimation (the admesh library).

Fixes "Unable to save project (#2445)"

src/admesh/util.cpp

@@ -457,3 +457,50 @@ All facets connected.  No further nearby check necessary.\n");
     stl_verify_neighbors(stl);
   }
 }
+
+// Check validity of the mesh, assert on error.
+bool stl_validate(stl_file *stl)
+{
+	assert(! stl->error);
+	assert(stl->fp == nullptr);
+	assert(stl->facet_start != nullptr);
+	assert(stl->heads == nullptr);
+	assert(stl->tail  == nullptr);
+	assert(stl->neighbors_start != nullptr);
+	assert((stl->v_indices == nullptr) == (stl->v_shared == nullptr));
+	assert(stl->stats.number_of_facets > 0);
+
+#ifdef _DEBUG
+    // Verify validity of neighborship data.
+    for (int facet_idx = 0; facet_idx < (int)stl->stats.number_of_facets; ++ facet_idx) {
+        const stl_neighbors &nbr 		= stl->neighbors_start[facet_idx];
+        const int 			*vertices 	= (stl->v_indices == nullptr) ? nullptr : stl->v_indices[facet_idx].vertex;
+        for (int nbr_idx = 0; nbr_idx < 3; ++ nbr_idx) {
+            int nbr_face = stl->neighbors_start[facet_idx].neighbor[nbr_idx];
+            assert(nbr_face < (int)stl->stats.number_of_facets);
+            if (nbr_face != -1) {
+            	int nbr_vnot = nbr.which_vertex_not[nbr_idx];
+				assert(nbr_vnot >= 0 && nbr_vnot < 6);
+				// Neighbor of the neighbor is the original face.
+				assert(stl->neighbors_start[nbr_face].neighbor[(nbr_vnot + 1) % 3] == facet_idx);
+				int vnot_back = stl->neighbors_start[nbr_face].which_vertex_not[(nbr_vnot + 1) % 3];
+				assert(vnot_back >= 0 && vnot_back < 6);
+				assert((nbr_vnot < 3) == (vnot_back < 3));
+				assert(vnot_back % 3 == (nbr_idx + 2) % 3);
+				if (vertices != nullptr) {
+					// Has shared vertices.
+	            	if (nbr_vnot < 3) {
+	            		// Faces facet_idx and nbr_face share two vertices accross the common edge. Faces are correctly oriented.
+						assert((stl->v_indices[nbr_face].vertex[(nbr_vnot + 1) % 3] == vertices[(nbr_idx + 1) % 3] && stl->v_indices[nbr_face].vertex[(nbr_vnot + 2) % 3] == vertices[nbr_idx]));
+					} else {
+	            		// Faces facet_idx and nbr_face share two vertices accross the common edge. Faces are incorrectly oriented, one of them is flipped.
+						assert((stl->v_indices[nbr_face].vertex[(nbr_vnot + 2) % 3] == vertices[(nbr_idx + 1) % 3] && stl->v_indices[nbr_face].vertex[(nbr_vnot + 1) % 3] == vertices[nbr_idx]));
+					}
+				}
+            }
+        }
+    }
+#endif /* _DEBUG */
+
+	return true;
+}