make autoscroll a bit easier to use, fix missing measure lines when moving the editor...

make autoscroll a bit easier to use, fix missing measure lines when moving the editor pane, slight optimisation(?) of canvas-simplerect (seems to make a difference to me), fix dirty pixels when moving regions across tracks sometimes. some code cleanup.

git-svn-id: svn://localhost/ardour2/trunk@885 d708f5d6-7413-0410-9779-e7cbd77b26cf