Use git submodule for bundled SDL2

[OlegOAndreev]

Currently sdl2-sys downloads SDL2-version.tar.gz from https://libsdl.org for every target when using "bundled" feature. This has two downsides:

1. Multiple copies of SDL2 are located in target folders.
2. Build process depends on external website.

Many other -sys crates use git submodules and include the C/C++ source code in the published crate. Now that SDL official repository was moved to Github, would you consider switching to using git submodule?

Currently SDL2 source size is around 80Mb. If you feel that the size bump is too big for users who do not use bundled feature, a separate crate could be used for bundled sources.

[Cobrand]

Yes, we could do that.