Upgrade to SDL 2.0.3

This upgrades our SDL2 renderer to 2.0.3, which adds support for (among other things) proper spaces integration on OSX:

![osx-spaces](https://cloud.githubusercontent.com/assets/167819/2631426/bb912848-be58-11e3-9a00-cdec9234cd49.png)

This also fixes the awkward file naming of the SDL2-CS wrapper.