Experimental/sdl2

Changes to make this branch working on Linux.
- SDL 2.0. seems to be working (but only with software driver)
- SDL version can be selected via ENABLE_SDL2 option in CMake.
- Due to usage of SDL_UpdateTexture minimum version of SDL is 2.0.1
- I haven't tested 1.2 compatibility due to some compile issues unrelated to my changes.