XInput over SDL

[Vincent Povirk]

>From what I could tell, SDL has a game controller api that's very
similar to xinput, which is pretty nice, and it can be configured to
map to any controller. So, although it is a heavy dependency, it could
get us a good solution very quickly.

Few other advantages:
 * The resulting xinput library could be used on Windows to support
non-xinput-compatible controllers.
 * Steam has a controller configuration GUI, which happens to be
compatible with SDL. The only alternative for picking up this
configuration would be to duplicate logic from SDL. (Admittedly, this
only helps for Wine-based ports of games on Steam, and possibly other
games when Steam is used as a launcher.)

We have very little control over heavy dependencies like SDL and can
end up with unfixable bugs. One immediate concern is that SDL sets up
signal handlers during initialization.

I'd be interested in seeing your code for this even without the cleanup.