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()); }