Support for unifying inner clip rectangle

[kvark]

Current implementation is limited by this bit:

                // TODO(gw): local inner is only valid if there's a single clip (for now).
                // This can be improved in the future, with some proper
                // rectangle region handling.
                if sources.len() > 1 {
                    local_inner = None;
                }

Removing it causes a failure in "inset-border-radius-ref.yaml". I don't think it will be difficult to investigate and fix.
cc @glennw

[kvark]

[glennw]

@kvark I think this is resolved by the recent clip changes by @mrobinson. Should we close this?