Refactor SDL to use liborbital
Created by: xTibor
This PR refactors the SDL port to use my new cbindgen-based Orbital client library, liborbital: https://github.com/xTibor/liborbital
After this change the functionality of the SDL port should remain the same, without any new (known) bugs or bugfixes.
If you find this PR okay, I could transfer that repo above to the Redox org as well.