Update SDL2 to 2.26.1

[res2k]

Hopefully addresses #231.

[apanteleev]

GitHub shows that the only differences between the two versions of SDL2 is the version number - is this right? How can that fix anything?

[res2k]

If you manually follow the submodule directory you get here: https://github.com/libsdl-org/SDL/tree/fd196c446a476282dd457fbafe9e9b3feae3f370
It's latest commit is from Apr 2020, and by inspecting e.g. configure.ac you can see this is SDL 2.0.13.

Not sure how GH computed that diff.
Above what's shows there's the message "fd196c446a476282dd457fbafe9e9b3feae3f370 and f17058b562c8a1090c0c996b42982721ace90903 are entirely different commit histories. " - I don't think it can provide anything sensible and maybe falls back to the last commit's diff.

[apanteleev]

I see, thanks. I should've looked at the diffs locally / more carefully.

Merged now.