panner: backwards compatibility, constrain pan-range in old sessions

fixes http://tracker.ardour.org/view.php?id=5823#c15686

This is only an issue when loading an old session
(pre 3.5.308 - actual change was 21ca6a10) for the first
time with newer ardour.
Saving and re-loading with ardour >= 3.5.308 fixed this as well.
Fix code to prepare re-enable cubic spline interpolation of automation data.

see also 24917e4c9e36ca57