Magnifier: Use a GUI::DisplayLink to drive the screen captures

This ensures that we don't try to update more often than the screen
is updated. It also avoids doing synchronous IPC in paint_event().

Userland/Applications/Magnifier/main.cpp

@@ -84,7 +84,6 @@ int main(int argc, char** argv)
     window->show();
 
     magnifier.track_cursor_globally();
-    magnifier.start_timer(16);
 
     return app->exec();
 }