Deprecate SDL 1.2

[Frenzie]

SDL 1.2 has been deprecated since 2014 or so.

https://wiki.libsdl.org/Support

SDL 1.2 is deprecated and not recommended for new projects.

Qemu recently followed suit: https://patchwork.kernel.org/patch/10164537/

I haven't actually tested it in years, so the status of SDL 1.2 in KOReader is that it may not be broken.

I think that other than the base CI, nothing really uses it anymore.

koreader-base/.travis.yml

Line 35 in 8e55e05

- libsdl1.2-dev

I briefly entertained the idea of testing both SDL 1.2 as well as SDL 2 (cf. koreader/koreader#4631 (comment)) but the additional maintenance burden isn't worth it.