Fix: Crash switching between sculpt and edit mode

Also fix a couple other places where normals layers weren't properly
tagged dirty or reallocated when the mesh changes.

Caused by cfa53e0fbe. When the size of a mesh changes,
the normal layers need to be reallocated. There were a couple of places
that cleared other runtime data with `BKE_mesh_runtime_clear_geometry`
but didn't deal with normals properly. Clearing the runtime "geometry"
is different from clearing the normals, because sometimes the size of
the normal layers doesn't have to change, in which case simply tagging
them dirty is fine.

source/blender/bmesh/intern/bmesh_mesh_convert.cc

@@ -948,9 +948,9 @@ void BM_mesh_bm_to_me(Main *bmain, BMesh *bm, Mesh *me, const struct BMeshToMesh
   CustomData_add_layer(&me->ldata, CD_MLOOP, CD_ASSIGN, mloop, me->totloop);
   CustomData_add_layer(&me->pdata, CD_MPOLY, CD_ASSIGN, mpoly, me->totpoly);
 
-  /* There is no way to tell if BMesh normals are dirty or not. Instead of calculating the normals
-   * on the BMesh possibly unnecessarily, just tag them dirty on the resulting mesh. */
-  BKE_mesh_normals_tag_dirty(me);
+  /* Clear normals on the mesh completely, since the original vertex and polygon count might be
+   * different than the BMesh's. */
+  BKE_mesh_clear_derived_normals(me);
 
   me->cd_flag = BM_mesh_cd_flag_from_bmesh(bm);
 

source/blender/editors/mesh/meshtools.c

@@ -428,6 +428,7 @@ int ED_mesh_join_objects_exec(bContext *C, wmOperator *op)
    * Even though this mesh wont typically have run-time data, the Python API can for e.g.
    * create loop-triangle cache here, which is confusing when left in the mesh, see: T90798. */
   BKE_mesh_runtime_clear_geometry(me);
+  BKE_mesh_clear_derived_normals(me);
 
   /* new material indices and material array */
   if (totmat) {

source/blender/makesrna/intern/rna_mesh.c

@@ -470,6 +470,7 @@ static void rna_MeshPolygon_flip(ID *id, MPoly *mp)
   BKE_mesh_polygon_flip(mp, me->mloop, &me->ldata);
   BKE_mesh_tessface_clear(me);
   BKE_mesh_runtime_clear_geometry(me);
+  BKE_mesh_normals_tag_dirty(me);
 }
 
 static void rna_MeshLoopTriangle_verts_get(PointerRNA *ptr, int *values)

source/blender/makesrna/intern/rna_mesh_api.c

@@ -183,7 +183,7 @@ static void rna_Mesh_flip_normals(Mesh *mesh)
 {
   BKE_mesh_polygons_flip(mesh->mpoly, mesh->mloop, &mesh->ldata, mesh->totpoly);
   BKE_mesh_tessface_clear(mesh);
-  BKE_mesh_calc_normals(mesh);
+  BKE_mesh_normals_tag_dirty(mesh);
   BKE_mesh_runtime_clear_geometry(mesh);
 
   DEG_id_tag_update(&mesh->id, 0);