Don't allow deletion of the first control point in automation lines

It seems to be an invariant that automation lines always have an
"anchor" control point at time zero. That control point is set when the
first real control point is set. It thus seems like it shouldn't be
possible to remove it unless the whole control set is being cleared..

However: If it really is an important invariant, it should be enforced
at a lower level. And it is also possible to drag the anchor away from
time zero, thus raising the question of when it is an anchor and why it
is needed. It seems like automation generally works fine without these
anchors.

gtk2_ardour/editor_mouse.cc

@@ -2338,6 +2338,11 @@ Editor::can_remove_control_point (ControlPoint& control_point)
 		if (line.is_last_point(control_point) || line.is_first_point(control_point)) {
 			return false;
 		}
+	} else {
+		/* we shouldn't remove the first automation point ... unless it also is the last */
+		if (line.is_first_point(control_point) && ! line.is_last_point(control_point)) {
+			return false;
+		}
 	}
 
 	return true;