Adds a way for Cogl to control event_mask of foreign wins

This extends cogl_onscreen_x11_set_foreign_xid to take a callback to a
function that details the event mask the Cogl requires the application
to select on foreign windows. This is required because Cogl, for
example, needs to track size changes of a window and may also in the
future want other notifications such as map/unmap.

Most applications wont need to use the foreign xwindow apis, but those
that do are required to pass a valid callback and update the event mask
of their window according to Cogl's requirements.

cogl/cogl-framebuffer.c

@@ -1624,9 +1624,17 @@ cogl_framebuffer_swap_region (CoglFramebuffer *framebuffer,
 #ifdef COGL_HAS_X11_SUPPORT
 void
 cogl_onscreen_x11_set_foreign_window_xid (CoglOnscreen *onscreen,
-                                          guint32 xid)
+                                          guint32 xid,
+                                          CoglOnscreenX11MaskCallback update,
+                                          void *user_data)
 {
+  /* We don't wan't applications to get away with being lazy here and not
+   * passing an update callback... */
+  g_return_if_fail (update);
+
   onscreen->foreign_xid = xid;
+  onscreen->foreign_update_mask_callback = update;
+  onscreen->foreign_update_mask_data = user_data;
 }
 
 guint32