Mesh: Move runtime data out of DNA

This commit replaces the `Mesh_Runtime` struct embedded in `Mesh`
with `blender::bke::MeshRuntime`. This has quite a few benefits:
- It's possible to use C++ types like `std::mutex`, `Array`,
  `BitVector`, etc. more easily
- Meshes saved in files are slightly smaller
- Copying and writing meshes is a bit more obvious without
  clearing of runtime data, etc.

The first is by far the most important. It will allows us to avoid a
bunch of manual memory management boilerplate that is error-prone and
annoying. It should also simplify future CoW improvements for runtime
data.

This patch doesn't change anything besides changing `mesh.runtime.data`
to `mesh.runtime->data`. The cleanups above will happen separately.

Differential Revision: https://developer.blender.org/D16180

source/blender/draw/intern/draw_manager_text.cc

@@ -15,6 +15,7 @@
 #include "BKE_editmesh.h"
 #include "BKE_editmesh_cache.h"
 #include "BKE_global.h"
+#include "BKE_mesh.h"
 #include "BKE_unit.h"
 
 #include "DNA_mesh_types.h"
@@ -233,8 +234,8 @@ void DRW_text_edit_mesh_measure_stats(ARegion *region,
   float clip_planes[4][4];
   /* allow for displaying shape keys and deform mods */
   BMIter iter;
-  const float(*vert_coords)[3] = (me->runtime.edit_data ? me->runtime.edit_data->vertexCos :
-                                                          nullptr);
+  const float(*vert_coords)[3] = (me->runtime->edit_data ? me->runtime->edit_data->vertexCos :
+                                                           nullptr);
   const bool use_coords = (vert_coords != nullptr);
 
   /* when 2 or more edge-info options are enabled, space apart */
@@ -339,8 +340,8 @@ void DRW_text_edit_mesh_measure_stats(ARegion *region,
     const float(*poly_normals)[3] = nullptr;
     if (use_coords) {
       BM_mesh_elem_index_ensure(em->bm, BM_VERT | BM_FACE);
-      BKE_editmesh_cache_ensure_poly_normals(em, me->runtime.edit_data);
-      poly_normals = me->runtime.edit_data->polyNos;
+      BKE_editmesh_cache_ensure_poly_normals(em, me->runtime->edit_data);
+      poly_normals = me->runtime->edit_data->polyNos;
     }
 
     BM_ITER_MESH (eed, &iter, em->bm, BM_EDGES_OF_MESH) {