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_cache_extract_mesh.cc

@@ -686,7 +686,7 @@ void mesh_buffer_cache_create_requested(struct TaskGraph *task_graph,
   MeshRenderData *mr = mesh_render_data_create(
       object, me, is_editmode, is_paint_mode, is_mode_active, obmat, do_final, do_uvedit, ts);
   mr->use_hide = use_hide;
-  mr->use_subsurf_fdots = mr->me && mr->me->runtime.subsurf_face_dot_tags != nullptr;
+  mr->use_subsurf_fdots = mr->me && mr->me->runtime->subsurf_face_dot_tags != nullptr;
   mr->use_final_mesh = do_final;
 
 #ifdef DEBUG_TIME