scrolledwindow: Add motion mask to the widget window

This is not specified specifically by the attached controllers, so let
the scrolledwindow set the mask, as motion events with no buttons pressed
are interesting to it.

https://bugzilla.gnome.org/show_bug.cgi?id=745344

gtk/gtkscrolledwindow.c

@@ -4041,7 +4041,7 @@ gtk_scrolled_window_realize (GtkWidget *widget)
   attributes.visual = gtk_widget_get_visual (widget);
   attributes_mask = GDK_WA_X | GDK_WA_Y | GDK_WA_VISUAL;
   attributes.event_mask = gtk_widget_get_events (widget) | GDK_EXPOSURE_MASK |
-    GDK_ENTER_NOTIFY_MASK | GDK_LEAVE_NOTIFY_MASK;
+    GDK_ENTER_NOTIFY_MASK | GDK_LEAVE_NOTIFY_MASK | GDK_POINTER_MOTION_MASK;
 
   window = gdk_window_new (gtk_widget_get_parent_window (widget),
                            &attributes, attributes_mask);