Added concurrent game window back-end
Added concurrent game window back-end
Merge branch 'master' of https://github.com/bvssvni/sdl2_game_window

Conflicts:
	src/concurrent_window_sdl2.rs
	src/lib.rs
Call `sdl2::init` to enable OpenGL 4.1

Closes https://github.com/PistonDevelopers/sdl2_game_window/issues/9
Merge pull request #10 from bvssvni/master

Call `sdl2::init` to enable OpenGL 4.1