add dedicated backend (WIP)

luigi/backend/ffisdl/sdl2/README.md

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+sdl2-ffi
+========
+
+A LuaJIT interface to SDL2
+
+# Installation #
+
+First, make sure SDL2 is installed on your system. This package only requires the binary shared libraries (.so, .dylib, .dll).
+Please see your package management system to install SDL2. You can also download yourself binaries on the
+[SDL2 web page](http://libsdl.org/download-2.0.php)
+
+```sh
+luarocks install https://raw.github.com/torch/sdl2-ffi/master/rocks/sdl2-scm-1.rockspec
+```
+
+*Note*: this SDL interface supports only SDL2, not SDL 1.2.
+
+# Usage #
+
+```lua
+local sdl = require 'sdl2'
+sdl.init(sdl.INIT_VIDEO)
+...
+```
+
+All SDL C functions are available in the `sdl` namespace returned by require. The only difference is the naming, which is not prefixed
+by `SDL_` anymore. The same goes for all C defines (like `SDL_INIT_VIDEO`, which can now be accessed with `sdl.INIT_VIDEO`).
+
+Although the interface is quite complete, there are still few defines not ported in this package. Fill free to post a message about it,
+or to request pulls.
+
+