Refactor GUI rendering model to be two-phased.

Instead of clients painting whenever they feel like it, we now ask that they
paint in response to a paint message.

After finishing painting, clients notify the WindowServer about the rect(s)
they painted into and then flush eventually happens, etc.

This stuff leaves us with a lot of badly named things. Need to fix that.

LibGUI/GEventLoop.cpp

@@ -134,7 +134,7 @@ void GEventLoop::wait_for_event()
         }
         switch (event.type) {
         case GUI_Event::Type::Paint:
-            dbgprintf("WID=%x Paint [%d,%d %dx%d]\n", event.window_id, event.paint.rect.location.x, event.paint.rect.location.y, event.paint.rect.size.width, event.paint.rect.size.height); break;
+            dbgprintf("WID=%x Paint [%d,%d %dx%d]\n", event.window_id, event.paint.rect.location.x, event.paint.rect.location.y, event.paint.rect.size.width, event.paint.rect.size.height);
             handle_paint_event(event, *window);
             break;
         case GUI_Event::Type::MouseDown:

LibGUI/GWidget.cpp

@@ -106,7 +106,7 @@ void GWidget::update()
     if (m_has_pending_paint_event)
         return;
     m_has_pending_paint_event = true;
-    GEventLoop::main().post_event(w, make<GPaintEvent>(relative_rect()));
+    w->update(relative_rect());
 }
 
 GWidget::HitTestResult GWidget::hit_test(int x, int y)

LibGUI/GWindow.cpp

@@ -49,11 +49,18 @@ GWindow::~GWindow()
 void GWindow::set_title(String&& title)
 {
     dbgprintf("GWindow::set_title \"%s\"\n", title.characters());
-    GUI_WindowParameters params;
     int rc = gui_set_window_title(m_window_id, title.characters(), title.length());
     ASSERT(rc == 0);
 }
 
+String GWindow::title() const
+{
+    char buffer[256];
+    int rc = gui_get_window_title(m_window_id, buffer, sizeof(buffer));
+    ASSERT(rc >= 0);
+    return String(buffer, rc);
+}
+
 void GWindow::set_rect(const Rect& a_rect)
 {
     dbgprintf("GWindow::set_rect! %d,%d %dx%d\n", a_rect.x(), a_rect.y(), a_rect.width(), a_rect.height());
@@ -85,7 +92,7 @@ void GWindow::event(GEvent& event)
             rect = m_main_widget->rect();
         m_main_widget->event(*make<GPaintEvent>(rect));
         GUI_Rect gui_rect = rect;
-        int rc = gui_invalidate_window(m_window_id, &gui_rect);
+        int rc = gui_notify_paint_finished(m_window_id, &gui_rect);
         ASSERT(rc == 0);
     }
 
@@ -105,9 +112,11 @@ void GWindow::show()
 {
 }
 
-void GWindow::update()
+void GWindow::update(const Rect& a_rect)
 {
-    GEventLoop::main().post_event(this, make<GPaintEvent>());
+    GUI_Rect rect = a_rect;
+    int rc = gui_invalidate_window(m_window_id, a_rect.is_null() ? nullptr : &rect);
+    ASSERT(rc == 0);
 }
 
 void GWindow::set_main_widget(GWidget* widget)

LibGUI/GWindow.h

@@ -16,6 +16,7 @@ public:
 
     int window_id() const { return m_window_id; }
 
+    String title() const;
     void set_title(String&&);
 
     int x() const { return rect().x(); }
@@ -40,7 +41,7 @@ public:
 
     void show();
 
-    void update();
+    void update(const Rect& = Rect());
 
 private:
     RetainPtr<GraphicsBitmap> m_backing;