This is a Win32-specific fix for bug 2726. A WinRT fix for this bug was applied
separately, via https://hg.libsdl.org/SDL/rev/91f56dcad879
This fix applies SDL_TOUCH_MOUSEID to 'mouse' events coming from touch devices,
but only when relative-mouse-mode is turned OFF. This bug is still present
when relative-mouse-mode is ON, however Microsoft does not provide documentation
on whether or not those input events (which come from WM_INPUT) can be
identified as touch-specific or not. Unofficially, that data might be available
(via GetMessageExtraInfo()), however this patch only uses MS-documented APIs.
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