WindowServer+Magnifier: Make Magnifier buttery smooth :^)

This patch moves the magnifier rect computation over to the server side
to ensure that the mouse cursor position and the screen image never get
out of sync.

Userland/Applications/Magnifier/MagnifierWidget.h

@@ -23,8 +23,6 @@ private:
 
     void sync();
 
-    Gfx::IntPoint m_mouse_position;
     int m_scale_factor { 2 };
-    int m_desktop_display_scale { 1 };
     RefPtr<Gfx::Bitmap> m_grabbed_bitmap;
 };