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?
It was simpler to just have the polling (actually: hotplug detection)
functions return immediately if it's not an appropriate time to poll.
Note that previously, if any joystick/controller was opened, we would poll
every time anyhow, skipping this function.
