the passed-in buffers valid. In the case where the main outs delivery is the last
processor in a route, this is not necessary (and wasteful). If another processor
(e.g. a meter) follows the main outs, the passed-in (scratch) buffers must be valid
or the meter will get garbage data.
Fixes meters displaying phantom signals in some cases.
git-svn-id: svn://localhost/ardour2/branches/3.0@6180 d708f5d6-7413-0410-9779-e7cbd77b26cf