compositor: fix focusing the stage window

We can't use the X11 stage window, if clutter is not using the X11
backend (and even if it was, it would be bogus when the xwayland
server is not the one clutter is talking to). Instead, we introduce
the concept of "focus type", which we use to differentiate the
various meanings of None in the focus_xwindow field.

https://bugzilla.gnome.org/show_bug.cgi?id=706364

src/core/display-private.h

@@ -86,6 +86,14 @@ typedef enum {
   META_TILE_MAXIMIZED
 } MetaTileMode;
 
+typedef enum {
+  META_FOCUS_NONE = 0,
+  META_FOCUS_X_CLIENT = 1,
+  META_FOCUS_WAYLAND_CLIENT = 2,
+  META_FOCUS_NO_FOCUS_WINDOW = 3,
+  META_FOCUS_STAGE = 4
+} MetaFocusType;
+
 struct _MetaDisplay
 {
   GObject parent_instance;
@@ -117,6 +125,7 @@ struct _MetaDisplay
    * like the no_focus_window or the stage X window. */
   Window focus_xwindow;
   gulong focus_serial;
+  MetaFocusType focus_type;
 
   /* last timestamp passed to XSetInputFocus */
   guint32 last_focus_time;
@@ -475,9 +484,10 @@ gboolean meta_display_process_barrier_event (MetaDisplay    *display,
                                              XIBarrierEvent *event);
 #endif /* HAVE_XI23 */
 
-void meta_display_set_input_focus_xwindow (MetaDisplay *display,
-                                           MetaScreen  *screen,
-                                           Window       window,
-                                           guint32      timestamp);
+void meta_display_set_input_focus_xwindow (MetaDisplay   *display,
+                                           MetaScreen    *screen,
+                                           MetaFocusType  type,
+                                           Window         window,
+                                           guint32        timestamp);
 
 #endif