core: update stale images AFTER checking focus

Partially revert 32754b0262.

We start with mapping the window (`win_process_update_flags()`). Then check
if focus has changed and process focus updates. Finally refresh stale images
(`win_process_image_flags`) because rules based on focus may have invalidated
them or require them to be created.

Fixes #465 with the following rule:
```
shadow-exclude = [
  "focused" != 1"
]
```

src/picom.c

@@ -1318,7 +1318,13 @@ static void handle_new_windows(session_t *ps) {
 
 static void refresh_windows(session_t *ps) {
 	win_stack_foreach_managed(w, &ps->window_stack) {
-		win_process_flags(ps, w);
+		win_process_update_flags(ps, w);
+	}
+}
+
+static void refresh_images(session_t *ps) {
+	win_stack_foreach_managed(w, &ps->window_stack) {
+		win_process_image_flags(ps, w);
 	}
 }
 
@@ -1359,7 +1365,7 @@ static void handle_pending_updates(EV_P_ struct session *ps) {
 		// stale.
 		handle_root_flags(ps);
 
-		// Process window flags
+		// Process window flags (window mapping)
 		refresh_windows(ps);
 
 		{
@@ -1371,6 +1377,9 @@ static void handle_pending_updates(EV_P_ struct session *ps) {
 			free(r);
 		}
 
+		// Process window flags (stale images)
+		refresh_images(ps);
+
 		e = xcb_request_check(ps->c, xcb_ungrab_server_checked(ps->c));
 		if (e) {
 			log_fatal_x_error(e, "failed to ungrab x server");

src/win.c

@@ -320,11 +320,15 @@ void win_release_images(struct backend_base *backend, struct managed_win *w) {
 	}
 }
 
-void win_process_flags(session_t *ps, struct managed_win *w) {
+void win_process_update_flags(session_t *ps, struct managed_win *w) {
 	if (win_check_flags_all(w, WIN_FLAGS_MAPPED)) {
 		map_win_start(ps, w);
 		win_clear_flags(w, WIN_FLAGS_MAPPED);
 	}
+}
+
+void win_process_image_flags(session_t *ps, struct managed_win *w) {
+	assert(!win_check_flags_all(w, WIN_FLAGS_MAPPED));
 
 	// Not a loop
 	while (win_check_flags_any(w, WIN_FLAGS_IMAGES_STALE) &&

src/win.h

@@ -254,8 +254,10 @@ struct managed_win {
 #endif
 };
 
-/// Process pending images flags on a window. Has to be called in X critical section
-void win_process_flags(session_t *ps, struct managed_win *w);
+/// Process pending updates/images flags on a window. Has to be called in X critical
+/// section
+void win_process_update_flags(session_t *ps, struct managed_win *w);
+void win_process_image_flags(session_t *ps, struct managed_win *w);
 /// Bind a shadow to the window, with color `c` and shadow kernel `kernel`
 bool win_bind_shadow(struct backend_base *b, struct managed_win *w, struct color c,
                      struct conv *kernel);