big rework of scrolling, horizontal part considered almost 100% done.

Many more changes than I would typically like in a single commit, but this was all very intertwined.
Vertical scrolling using track-stepping still to follow.

libs/canvas/curve.cc

@@ -402,7 +402,7 @@ Curve::render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const
 		context->restore ();
 	}
 
-#if 0
+#if 1
 	/* add points */
 	
 	setup_fill_context (context);