WIP: Adding support for SDL2

This is WIP of SDL2 support. Just install SDL2 libraries, configure should detect first SDL2.
There is some issue with texts for OpenGL for some cases for Grim engine, It seems it's specific to my GL drivers, I don't know what is source of it.
It's not tested very well, I would like others look into code and try runtime.
So comments and suggestions are welcome.

I made changes to set before creating window, I'll look into later why that is needed.