viewport: select for touch events

This makes kinetic scrolling work with viewports where the
content does not otherwise select for button or touch events,
such as testscrolledwindow's label.

gtk/gtkviewport.c

@@ -682,7 +682,7 @@ gtk_viewport_realize (GtkWidget *widget)
   event_mask = gtk_widget_get_events (widget) | GDK_EXPOSURE_MASK;
   /* We select on button_press_mask so that button 4-5 scrolls are trapped.
    */
-  attributes.event_mask = event_mask | GDK_BUTTON_PRESS_MASK;
+  attributes.event_mask = event_mask | GDK_BUTTON_PRESS_MASK | GDK_TOUCH_MASK;
 
   attributes_mask = GDK_WA_X | GDK_WA_Y | GDK_WA_VISUAL;