Missing KMOD_SCROLL

[this-is-your-do]

While SDL2 supports querying the current state of the CapsLock & NumLock toggle via SDL_GetModState with KMOD_CAPS and KMOD_NUM - it doesn't support querying the current state of the ScrollLock toggle.

Thus, it's not currently possible to know its state currently (you can't just listen to key press events, as its state may change before your app is launched, as an example [or when the app isn't in focus, as another])

Now, ScrollLock is certainly one of the less popular keys, but it still exists on most "standard" desktop keyboards, and common OSes like linux & windows handle its toggling just like they do with CapsLock & NumLock.

So it would be great if SDL exposed this as well, just like it already exposes CapsLock & NumLock.

Thanks!