Linux support and some bugfixes

Hi!

I stumbled upon your opengl gui for gnu rocket when trying to port the qt editor from the official repo initially to linux, and realized that your opengl-based editor is much better. It was only lacking support for my computer.

I fixed some case sensitivity problems in header includes, wrote preliminary SDL-based support for linux, and found & fixed two minor bugs while hacking.

Pull and let other linux users know about this. I tried to adhere to your coding style, but do complain if something is way off.