Handle changed session duration & resizing canvas better wrt end marker, remove unuse...

Handle changed session duration & resizing canvas better wrt end marker, remove unused last_canvas_frame, don't redraw the entire mixer every time its presented, consistent space between rightmost horizontal scroller position and end marker when zooming, possible fix for gaps in tempo lines.

git-svn-id: svn://localhost/ardour2/branches/3.0@3982 d708f5d6-7413-0410-9779-e7cbd77b26cf