Video::gl_get_swap_interval() should return an enum

[icefoxen]

Right now it returns an i32 when the only valid values are 1, 0, and -1, all of which have specific discrete meanings.

Naturally, Video::gl_set_swap_interval() should also use this enum.

Also it'd be nice if "vsync" were mentioned in the docs near it so you could ever find it. Grrrr SDL2.

Will throw together a PR when I get a chance.

[Cobrand]

Closed by #684