gl_set_swap_interval doesn't support "long" intervals

[SolraBizna]

A little-known, little-used, apparently undocumented feature of SDL_GL_SetSwapInterval is that (on some systems, at least) you can meaningfully provide a value greater than 1. For example, a value of 2 syncs to every second vertical blank, which on a 60Hz display translates to 30Hz.

I used this in a previous iteration of a project that I am reimplementing in Rust—though I have coincidentally eliminated the feature that made use of "long" swap intervals. I'm not aware of any other usage, and since my project is extremely Linux-specific I have no idea whether or not it can be expected to work on macOS or Windows.

rust-sdl2 doesn't support this usage. The question is, does anyone actually care? (Heck, I'm not even sure I care.)

[Cobrand]

If this is the SDL2 offers, we want to support it the best we can. We are only excluding things that are truly useless from this crate (for instance, the timer utility, the network utility, ...), everything else we will want to support it especially if there is no other rust alternative to do the same thing.

Thanks for noticing, and as usual, PRs welcome ;)