subtle change in event handling for MidiViews

this needs more investigation but we need to return false from
scroll event handling in order for stuff to work. this should
not be the case and this will likely be reverted once the reason
it is required is uncovered

gtk2_ardour/midi_view.cc

@@ -413,7 +413,7 @@ MidiView::midi_canvas_group_event (GdkEvent* ev)
 
 	case GDK_SCROLL:
 		if (scroll (&ev->scroll)) {
-			return true;
+			return false;
 		}
 		break;