Bug fix #99: Rewrite focus detection logic

- Rewrite focus detection logic. Remove w->focused_real and use
  ps->active_win to identify focused window uniformly. Use a more
  expensive way to filter FocusIn/Out events to improve reliability.
  Only limited tests are done, and bugs are likely to be introduced.
  (#99)

- Known issue: Under fvwm, compton sometimes does not consistently
  report the window input gets sent to. But there's something wrong in
  that case: XGetInputFocus() shows the root window is focused but
  another window is receiving input.

src/compton.h

@@ -563,6 +563,20 @@ clear_cache_win_leaders(session_t *ps) {
 static win *
 find_toplevel2(session_t *ps, Window wid);
 
+/**
+ * Find matched window.
+ */
+static inline win *
+find_win_all(session_t *ps, const Window wid) {
+  if (!wid || PointerRoot == wid || wid == ps->root || wid == ps->overlay)
+    return NULL;
+
+  win *w = find_win(ps, wid);
+  if (!w) w = find_toplevel(ps, wid);
+  if (!w) w = find_toplevel2(ps, wid);
+  return w;
+}
+
 static Window
 win_get_leader_raw(session_t *ps, win *w, int recursions);
 
@@ -589,7 +603,7 @@ group_is_focused(session_t *ps, Window leader) {
 
   for (win *w = ps->list; w; w = w->next) {
     if (win_get_leader(ps, w) == leader && !w->destroyed
-        && w->focused_real)
+        && win_is_focused_real(ps, w))
       return true;
   }
 
@@ -764,6 +778,9 @@ group_update_focused(session_t *ps, Window leader) {
 static inline void
 win_set_focused(session_t *ps, win *w, bool focused);
 
+static void
+win_on_focus_change(session_t *ps, win *w);
+
 static void
 win_determine_fade(session_t *ps, win *w);