clay/renderers/SDL2

Please note, the SDL2 renderer is not 100% feature complete. It is currently missing:

- Rounded rectangle corners

Note: on Mac OSX, SDL2 for some reason decides to automatically disable momentum scrolling on macbook trackpads.
You can re enable it in objective C using:

```C
[[NSUserDefaults standardUserDefaults] setBool: YES
                                       forKey: @"AppleMomentumScrollSupported"];
```