likely fix for cases where a button release event falls through from the canvas to the editor, during a drag

The drag code expects coordinates in canvas coordinates, but we were not translating them at the editor level (canvas event handling
does do this, but cannot affect the situation if the event falls through to the editor)

gtk2_ardour/editor_canvas_events.cc

@@ -188,7 +188,15 @@ Editor::track_canvas_button_release_event (GdkEventButton *event)
 {
 	if (!Keyboard::is_context_menu_event (event)) {
 		if (_drags->active ()) {
-			_drags->end_grab ((GdkEvent*) event);
+
+			GdkEvent copy = *((GdkEvent*) event);
+			Duple winpos = Duple (event->x, event->y);
+			Duple where = _track_canvas->window_to_canvas (winpos);
+
+			copy.button.x = where.x;
+			copy.button.y = where.y;
+
+			_drags->end_grab (&copy);
 		}
 	}
 	return false;