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/modifiers/intern/MOD_wave.cc

@@ -21,6 +21,7 @@
 #include "BKE_context.h"
 #include "BKE_deform.h"
 #include "BKE_editmesh.h"
+#include "BKE_editmesh_cache.h"
 #include "BKE_lib_id.h"
 #include "BKE_lib_query.h"
 #include "BKE_mesh.h"
@@ -337,7 +338,7 @@ static void deformVertsEM(ModifierData *md,
 
   if (!ELEM(mesh_src, nullptr, mesh)) {
     /* Important not to free `vertexCos` owned by the caller. */
-    EditMeshData *edit_data = mesh_src->runtime.edit_data;
+    EditMeshData *edit_data = mesh_src->runtime->edit_data;
     if (edit_data->vertexCos == vertexCos) {
       edit_data->vertexCos = nullptr;
     }