fix clamping of line and rect coordinates to avoid issues with cairo when drawing way outside a surface's dimensions; move various coordinate methods down to Canvas, because they don't require GTK information; make visible_area() a Canvas virtual method so that we don't have to cast to call it
remove all calls to Outline::set_outline_width (1) because this is "wrong" when using Cairo - the default is 0.5 and it can remain that way
Merge branch 'cairocanvas' of git.ardour.org:ardour/ardour into cairocanvas