Fix: Hide corner vert and edge attributes in UI

These are meant to be like other low-level attributes--
hidden from the UI, at least by default. This was just
missing from the initial commit that added these
because seeing them was useful for debugging.

source/blender/blenkernel/intern/attribute_access.cc

@@ -54,6 +54,9 @@ const char *no_procedural_access_message =
 
 bool allow_procedural_attribute_access(StringRef attribute_name)
 {
+  if (attribute_name.startswith(".corner")) {
+    return false;
+  }
   if (attribute_name.startswith(".select")) {
     return false;
   }