Merge pull request #30 from bvssvni/master

Added gfx-rs support
Merge pull request #31 from bvssvni/master

Updated to latest gfx-rs
Merge pull request #32 from bvssvni/master

Added Retina support
Removed support for `ConcurrentGame`

Closes https://github.com/PistonDevelopers/sdl2_game_window/issues/33
Removed support for `ConcurrentGame`