remove platform dependent unused stuff from sdl_bindings.rs, update sdl and dependencies

[antonilol]

• update sdl2
• remove platform dependent unused stuff from sdl_bindings.rs
• update bindgen
• fix ci? -> later pr
• check generated sdl_bindings.rs in ci? -> later pr or maybe not

closes #1496
closes #1500

[antonilol]

I set it to ready to review earlier, but now it is actually ready.

I also have a suspicion as to why the CI fails, at least only the windows case (not the vcpkg case):
Some X11 event structures are checked by bindgen on windows, from bindings generated on linux. This did not happen before because bindgen used to generate "runtime tests" using #[test] fn test() {}, but now does "compile time tests" that make the compilation fail if the layout of the structures is not expected. I guess a pointer width difference makes it fail?

[antonilol]

56a65ed is a small otherwise unrelated change I added (I need Default for keyboard::Mod). It is documented in the changelog as well.

[jagprog5]

looks fine to me. @Cobrand

[iddm]

Hey guys! Thank you for your great work! Can you also release this with a minor version update (0.38.x)?

[antonilol]

I'd rather fix the build issue than backport this, and then have these changes in 0.39. What do you think?