WinRT build fix
WinRT: Fixed game controller axis mappings

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?