[MERGE] Add wrapper function for lazily registering filter stack maps.

[Ian Clatworthy]

Ian Clatworthy has voted approve.
Status is now: Approved
Comment:
My preference is to keep the registry variable private for now and 
mutate it via functions like you've done.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C20090323031217.5E8DB1D358%40ganieda.vernstok.nl%3E
Project: Bazaar