Fix problems with selecting control points in AutomationRegionViews with non-zero...

Fix problems with selecting control points in AutomationRegionViews with non-zero region start.  Firstly, determine_visible_control_points considers a subset of the AutomationLine's points if start is non-zero, and this must be taken into account when matching up AutomationLine points with AutomationList points.  Secondly, _offset should not part of the calculation when converting model position to session frames.  Fixes #4338.

git-svn-id: svn://localhost/ardour2/branches/3.0@10151 d708f5d6-7413-0410-9779-e7cbd77b26cf