SDL/WinRT currently uses a separate XInput backend from SDL/Win32, as WinRT
has no support for DirectInput. This change makes SDL/WinRT's XInput
code snag some recently-changed bits from the Win32-specific,
DirectInput + XInput backend, in order to get the SDL_GameController API
working again on WinRT, insofar that axes map to the correct parts.
TODO:
- test all buttons, making sure WinRT maps buttons the same way that Win32 does
- consider making the Win32 and WinRT codebases share more stuff, minus
the sort of duplication happening via this change. Maybe simulate, or
stub-out, DirectInput calls when on WinRT?
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