Mesh: Start implementing mesh version of triangulation

Issues with multithreading still

@@ -177,0 +181,4 @@
+                                                 const float v4[3],
+                                                 const bool lock_degenerate)
+{
+  /* not a loop (only to be able to break out) */

@@ -0,0 +300,4 @@
+                              const OffsetIndices<int> faces,
+                              const int edges_start,
+                              const int corner_map_offset,
+                              MutableSpan<int> corner_map,

@@ -0,0 +476,4 @@
+  });
+}
+
+std::optional<Mesh *> mesh_triangulate(

@@ -0,0 +14,4 @@
+
+namespace blender::geometry {
+
+enum class TriangulateNGonMode {

@@ -0,0 +30,4 @@
+std::optional<Mesh *> mesh_triangulate(
+    const Mesh &src_mesh,
+    const IndexMask &selection,
+    const TriangulateNGonMode ngon_mode,

@@ -0,0 +518,4 @@
+  const OffsetIndices edges_by_ngon = calc_edges_by_ngon(src_faces, ngons, edge_offset_data);
+
+  /* Unselected faces are moved to the beginning of the new mesh. */
+  const IndexRange unselected_range(0, unselected.size());

@@ -0,0 +535,4 @@
+  /* All newly created edges (from Ngons and quads). */
+  const IndexRange inner_edges(ngon_edges.start(), ngon_edges.size() + quad_edges.size());
+
+  /* Create a mesh with no face corners, since we don't know how many there will be yet. */

@@ -97,0 +62,4 @@
+    std::optional<Mesh *> mesh = geometry::mesh_triangulate(
+        *src_mesh,
+        selection,
+        geometry::TriangulateNGonMode(ngon_method),

@@ -180,3 +179,1 @@
- * \note The result may be an empty span.
- */
-static MutableSpan<int> get_orig_index_layer(Mesh &mesh, const eAttrDomain domain)
+static std::optional<MutableSpan<int>> get_orig_index_layer(Mesh &mesh, const eAttrDomain domain)