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.

examples/x11-foreign.c

@@ -5,6 +5,24 @@
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
 
+static void
+update_cogl_x11_event_mask (CoglOnscreen *onscreen,
+                            guint32 event_mask,
+                            void *user_data)
+{
+  XDisplay *xdpy = user_data;
+  XSetWindowAttributes attrs;
+  guint32 xwin;
+
+  attrs.event_mask = event_mask;
+  xwin = cogl_onscreen_x11_get_window_xid (onscreen);
+
+  XChangeWindowAttributes (xdpy,
+                           (Window)xwin,
+                           CWEventMask,
+                           &attrs);
+}
+
 int
 main (int argc, char **argv)
 {
@@ -110,7 +128,9 @@ main (int argc, char **argv)
 
   XFree (xvisinfo);
 
-  cogl_onscreen_x11_set_foreign_window_xid (onscreen, xwin);
+  cogl_onscreen_x11_set_foreign_window_xid (onscreen, xwin,
+                                            update_cogl_x11_event_mask,
+                                            xdpy);
 
   fb = COGL_FRAMEBUFFER (onscreen);
   /* Eventually there will be an implicit allocate on first use so this