WIP: gallium: Drop post-processing filters
As far as I can tell, nobody uses these; they're only exposed through user drirc's which are generally unused these days anyway. At any rate, I really don't think Gallium is the right place to maintain these sorts of effects; doing them correctly is the app's domain (or the compositor's).
Let's drop the legacy code and lighten the tree.
If there is a use-case I'm not aware of please let me know.