Fading in/out of black which switching video modes

Hey,

This is the first time I've contributed to a C++ project (I'm relatively new, so be gentle).

I develop on OSX, and when entering full screen mode at a different resolution, we get this ugly French Blue color fade in and out.

Libraries like SDL combat this color by customising that color to be something like black. This patch attempts to replicate that behaviour. On OSX, when entering full screen mode, your screen should fade nicely between the desktop and your application.

Thanks!

Keith