WSCompositor: Use a timer to schedule compose rather than an event

Really poor man's vsync. Still not actual vsync, but at least we won't
constantly spin buffers if we get many dirty rects.

Servers/WindowServer/WSCompositor.cpp

@@ -8,6 +8,8 @@
 #include <SharedGraphics/PNGLoader.h>
 #include <SharedGraphics/Painter.h>
 
+// #define COMPOSITOR_DEBUG
+
 WSCompositor& WSCompositor::the()
 {
     static WSCompositor s_the;
@@ -25,15 +27,15 @@ WSCompositor::WSCompositor()
 
     m_wallpaper_path = "/res/wallpapers/retro.rgb";
     m_wallpaper = GraphicsBitmap::load_from_file(GraphicsBitmap::Format::RGBA32, m_wallpaper_path, { 1024, 768 });
-}
 
-void WSCompositor::event(CEvent& event)
-{
-    if (event.type() == WSEvent::WM_DeferredCompose) {
-        m_pending_compose_event = false;
+    m_compose_timer.on_timeout = [=]() {
+#if defined(COMPOSITOR_DEBUG)
+        dbgprintf("WSCompositor: frame callback\n");
+#endif
         compose();
-        return;
-    }
+    };
+    m_compose_timer.set_single_shot(true);
+    m_compose_timer.set_interval(1000 / 60);
 }
 
 void WSCompositor::compose()
@@ -159,12 +161,11 @@ void WSCompositor::invalidate(const Rect& a_rect)
     if (rect.is_empty())
         return;
 
+#if defined(COMPOSITOR_DEBUG)
+    dbgprintf("Invalidated: %dx%d %dx%d\n", a_rect.x(), a_rect.y(), a_rect.width(), a_rect.height());
+#endif
     m_dirty_rects.add(rect);
-
-    if (!m_pending_compose_event) {
-        WSEventLoop::the().post_event(*this, make<WSEvent>(WSEvent::WM_DeferredCompose));
-        m_pending_compose_event = true;
-    }
+    m_compose_timer.start();
 }
 
 bool WSCompositor::set_wallpaper(const String& path, Function<void(bool)>&& callback)

Servers/WindowServer/WSCompositor.h

@@ -3,6 +3,7 @@
 #include <AK/OwnPtr.h>
 #include <AK/RetainPtr.h>
 #include <LibCore/CObject.h>
+#include <LibCore/CTimer.h>
 #include <SharedGraphics/DisjointRectSet.h>
 #include <SharedGraphics/GraphicsBitmap.h>
 
@@ -26,7 +27,6 @@ public:
     Rect current_cursor_rect() const;
 
 private:
-    virtual void event(CEvent&) override;
     virtual const char* class_name() const override { return "WSCompositor"; }
 
     WSCompositor();
@@ -39,7 +39,7 @@ private:
 
     unsigned m_compose_count { 0 };
     unsigned m_flush_count { 0 };
-    bool m_pending_compose_event { false };
+    CTimer m_compose_timer;
     bool m_flash_flush { false };
     bool m_buffers_are_flipped { false };