Include SDL as a submodule

Currently, the entire SDL2 2.0.8 library is included in the repo, including its artifacts.

This is stupid and wrong. Also, SDL2 has line-draw primitives in it, which entirely misses the point of the project.

Better to add SDL1 as a submodule and revert the port to SDL2.

Inspect the repository's current SDL2 copy and the code changed by the SDL2 port to identify the dependency and integration entry points. Replace the bundled SDL2 with an SDL1 submodule and revert the SDL2 port; done means SDL2 artifacts are no longer included and the project uses SDL1.

Written by the indexing model from the issue text.