WindowServer: Try harder to avoid re-rendering window shadow

We only really need to re-render the simple window shadow when
the size of the frame changes. So, for all other cases only re-render
the window frame without rendering the shadow.

Userland/Services/WindowServer/WindowFrame.h

@@ -113,6 +113,7 @@ private:
     size_t m_flash_counter { 0 };
     float m_opacity { 1 };
     bool m_has_alpha_channel { false };
+    bool m_shadow_dirty { false };
     bool m_dirty { false };
 };