native: Add SDL3 WSI backend

[flibitijibibo]

SDL 3.1.3 was released today, and the ABI has been frozen! Making an SDL3 backend should now be safe - we can copypaste most of SDL2 and just fix the small bits that dxvk depends on (I think only monitor enumeration changed significantly, everything else is pretty much the same).

[doitsujin]

Sounds good, currently drowned in work but this is the kind of thing we should be able to do before the next release.