WindowServer: Spawn a secondary thread to decode wallpapers.

The threading API's are not very mature, so this code looks a bit crufty
but it does take the load off the WindowServer main thread when changing
wallpapers. :^)

Servers/WindowServer/WSClientConnection.cpp

@@ -332,11 +332,12 @@ void WSClientConnection::handle_request(const WSAPISetWindowOpacityRequest& requ
 
 void WSClientConnection::handle_request(const WSAPISetWallpaperRequest& request)
 {
-    bool success = WSWindowManager::the().set_wallpaper(request.wallpaper());
-    WSAPI_ServerMessage response;
-    response.type = WSAPI_ServerMessage::Type::DidSetWallpaper;
-    response.value = success;
-    post_message(response);
+    WSWindowManager::the().set_wallpaper(request.wallpaper(), [&] (bool success) {
+        WSAPI_ServerMessage response;
+        response.type = WSAPI_ServerMessage::Type::DidSetWallpaper;
+        response.value = success;
+        post_message(response);
+    });
 }
 
 void WSClientConnection::handle_request(const WSAPIGetWallpaperRequest&)

Servers/WindowServer/WSWindowManager.cpp

@@ -188,16 +188,42 @@ void WSWindowManager::tick_clock()
     invalidate(menubar_rect());
 }
 
-bool WSWindowManager::set_wallpaper(const String& path)
+bool WSWindowManager::set_wallpaper(const String& path, Function<void(bool)>&& callback)
 {
-    auto bitmap = load_png(path);
-    if (!bitmap)
-        return false;
+    struct Context {
+        String path;
+        RetainPtr<GraphicsBitmap> bitmap;
+        Function<void(bool)> callback;
+    };
+    auto context = make<Context>();
+    context->path = path;
+    context->callback = move(callback);
 
+    int rc = create_thread([] (void* ctx) -> int {
+        OwnPtr<Context> context((Context*)ctx);
+        context->bitmap = load_png(context->path);
+        if (!context->bitmap) {
+            context->callback(false);
+            exit_thread(0);
+            return 0;
+        }
+        the().deferred_invoke([context = move(context)] (auto&) {
+            the().finish_setting_wallpaper(context->path, *context->bitmap);
+            context->callback(true);
+        });
+        exit_thread(0);
+        return 0;
+    }, context.leak_ptr());
+    ASSERT(rc == 0);
+
+    return true;
+}
+
+void WSWindowManager::finish_setting_wallpaper(const String& path, Retained<GraphicsBitmap>&& bitmap)
+{
     m_wallpaper_path = path;
     m_wallpaper = move(bitmap);
     invalidate();
-    return true;
 }
 
 void WSWindowManager::set_resolution(int width, int height)

Servers/WindowServer/WSWindowManager.h

@@ -85,7 +85,7 @@ public:
 
     void set_resolution(int width, int height);
 
-    bool set_wallpaper(const String& path);
+    bool set_wallpaper(const String& path, Function<void(bool)>&& callback);
     String wallpaper_path() const { return m_wallpaper_path; }
 
     const WSCursor& active_cursor() const;
@@ -144,6 +144,7 @@ private:
     void tell_wm_listener_about_window_icon(WSWindow& listener, WSWindow&);
     void tell_wm_listener_about_window_rect(WSWindow& listener, WSWindow&);
     void pick_new_active_window();
+    void finish_setting_wallpaper(const String& path, Retained<GraphicsBitmap>&&);
 
     WSScreen& m_screen;
     Rect m_screen_rect;