SDL2 bindings for Crystal

Write SDL2 games in Crystal. Support for desktop applications for now, but Android shouldn't be hard to achieve.

Most of the bindings have been implemented, though most haven't been verified, yet. Please see the samples directory for examples, ported from the tutorials found at http://lazyfoo.net/tutorials/SDL/index.php. You are welcome to port more samples, and the necessary corrections!

Requirements

• SDL2 is required;
• Optional bindings for SDL2_Mixer, SDL2_Image and SDL2_TTF;
• Crystal > 0.22.0 is required for SDL2_Mixer to work correctly.