Sculpt: external file storage for multires

* This is experimental, the file format may change still!
* Helps reduce memory usage, keeps .blend files smaller, and makes
  saving quicker when not editing multires.
* This is implemented at the customdata level, currently only the
  multires displacements can be stored externally.

ToDo

* Better integration with object duplication/removal/..
* Memory is not yet freed when exiting sculpt mode.
* Loading only lower levels is not supported yet.

release/scripts/ui/properties_data_modifier.py

@@ -302,9 +302,9 @@ class DATA_PT_modifiers(DataButtonsPanel):
         col = split.column()
         col.itemO("object.multires_subdivide", text="Subdivide")
         col.itemO("object.multires_higher_levels_delete", text="Delete Higher")
-        # row = col.row()
-        # row.enabled = md.total_levels > 0
-        # row.itemR(md, "external")
+        row = col.row()
+        row.enabled = md.total_levels > 0
+        row.itemR(md, "external")
 
     def PARTICLE_INSTANCE(self, layout, ob, md):
         layout.itemR(md, "object")