Added sending a resize event when the scaling factor changes on OS X

This is a small bug fix that triggers a resize event when the scaling factor changes on OS X, such as when moving a window between a low-DPI display and a Retina display. The relevant forum discussion is [here](http://en.sfml-dev.org/forums/index.php?topic=17050), and a small example program that can be used to demonstrate the issue is [here](https://gist.github.com/tomgalvin594/ab67b962cf10c809cb68).