Fixed bug 2726 - WinRT touches not setting 'which' field in virtual mouse events

This patch makes sure that any SDL_MOUSEBUTTONDOWN, SDL_MOUSEBUTTONUP, and
SDL_MOUSEMOTION events, as triggered by a touch event in a WinRT app, set the
event's 'which' field to SDL_TOUCH_MOUSEID.  Previously, this was getting set
to the same value as events from a real mouse, '0'.

Thanks to Diego for providing information on this bug, and to Tamas Hamor for
sending over a patch!