Include static libraries in project

[gen2brain]

This PR add a static tag and libraries that allows one to build a standalone binary on Linux/Windows/macOS. Discussed in #329 .

[veeableful]

Hi @gen2brain, I have set up a repository that stores all the static libraries in https://github.com/veandco/go-sdl2-libs. I figured that since it takes about 37M, it would be better to make .go-sdl2-libs a submodule. Could you perhaps change it to submodule before I merge? That way, I think this repository will not store the files in history.

As a side note, I've also invited you to become a collaborator as well on the projects since you have done so much. Please join us! 😁

[gen2brain]

Hi @veeableful, I changed libs to submodule. Thanks for invite! I think I can prepare some script to update SDL libs in future, and it should probably be done on some old Ubuntu, e.g. 14.04.

Also, I will see if I can update https://github.com/gen2brain/go-sdl2-android-example.

[veeableful]

Thanks @gen2brain!