Mesh: skip conversion from edit-mesh to mesh in edit-mode

This resolves a performance regression in 2.8x where every edit-mode
update performed an edit-mesh to mesh conversion.

Now the conversion will be lazily initialized if/when it's required.

New BKE_mesh_wrapper_* functions abstract over mesh data access.
Currently only edit-mesh and regular meshes are supported.
In the future sub-surface meshes may be supported too.

source/blender/blenkernel/BKE_editmesh_cache.h

@@ -33,6 +33,11 @@ void BKE_editmesh_cache_ensure_vert_normals(struct BMEditMesh *em, struct EditMe
 
 void BKE_editmesh_cache_ensure_poly_centers(struct BMEditMesh *em, struct EditMeshData *emd);
 
+bool BKE_editmesh_cache_calc_minmax(struct BMEditMesh *em,
+                                    struct EditMeshData *emd,
+                                    float min[3],
+                                    float max[3]);
+
 #ifdef __cplusplus
 }
 #endif