SharedGraphics: Add PainterStateSaver RAII helper and Point::operator-().

Two little things to help tidy up a bit in WSWindowManager.

SharedGraphics/Painter.h

@@ -80,3 +80,20 @@ private:
     Retained<GraphicsBitmap> m_target;
     Vector<State> m_state_stack;
 };
+
+class PainterStateSaver {
+public:
+    PainterStateSaver(Painter& painter)
+        : m_painter(painter)
+    {
+        m_painter.save();
+    }
+
+    ~PainterStateSaver()
+    {
+        m_painter.restore();
+    }
+
+private:
+    Painter& m_painter;
+};

SharedGraphics/Point.h

@@ -48,6 +48,8 @@ public:
         return !(*this == other);
     }
 
+    Point operator-() const { return { -m_x, -m_y }; }
+
     operator WSAPI_Point() const;
     String to_string() const { return String::format("[%d,%d]", x(), y()); }
 

WindowServer/WSWindowManager.cpp

@@ -876,24 +876,20 @@ void WSWindowManager::compose()
         if (!any_dirty_rect_intersects_window(window))
             return;
         for (auto& dirty_rect : dirty_rects.rects()) {
+            PainterStateSaver saver(*m_back_painter);
             m_back_painter->set_clip_rect(dirty_rect);
             paint_window_frame(window);
             Rect dirty_rect_in_window_coordinates = Rect::intersection(dirty_rect, window.rect());
-            if (dirty_rect_in_window_coordinates.is_empty()) {
+            if (dirty_rect_in_window_coordinates.is_empty())
-                m_back_painter->clear_clip_rect();
                 continue;
-            }
+            dirty_rect_in_window_coordinates.move_by(-window.position());
-            dirty_rect_in_window_coordinates.set_x(dirty_rect_in_window_coordinates.x() - window.x());
-            dirty_rect_in_window_coordinates.set_y(dirty_rect_in_window_coordinates.y() - window.y());
             auto dst = window.position();
             dst.move_by(dirty_rect_in_window_coordinates.location());
             if (window.opacity() == 1.0f)
                 m_back_painter->blit(dst, *backing_store, dirty_rect_in_window_coordinates);
             else
                 m_back_painter->blit_with_opacity(dst, *backing_store, dirty_rect_in_window_coordinates, window.opacity());
-            m_back_painter->clear_clip_rect();
         }
-        m_back_painter->clear_clip_rect();
     };
 
     for_each_visible_window_from_back_to_front([&] (WSWindow& window) {