WinRT: hiding SDL_WinRTRunApp's XAML functionality, until it's ready for use

SDL_WinRTRunApp() is used on WinRT to launch a main(int, char **)-style
function.  It has optional, and experimental support for launching content
inside a XAML control, backed by a main() function running on a separate thread.
This is provided via it's 2nd parameter, which can be a pointer to a XAML
control.  (If NULL, XAML support will not be used.)

This change renames the experimental feature's parameter (to SDL_WinRTRunApp())
as "reserved", until such time as the functionality is ready for use.  It will
likely be renamed again in the future, when running SDL on top of a XAML control
via a separate thread, becomes reasonably usable.

include/SDL_main.h

@@ -142,14 +142,11 @@ extern DECLSPEC void SDLCALL SDL_UnregisterApp(void);
  *  \brief Initializes and launches an SDL/WinRT application.
  *
  *  \param mainFunction The SDL app's C-style main().
- *  \param xamlBackgroundPanel An optional, XAML-based, background panel.
- *     For Non-XAML apps, this value must be set to NULL.  For XAML apps,
- *     pass in a pointer to a SwapChainBackgroundPanel, casted to an
- *     IInspectable (via reinterpret_cast).
+ *  \param reserved Reserved for future use; should be NULL
  *  \ret 0 on success, -1 on failure.  On failure, use SDL_GetError to retrieve more
  *      information on the failure.
  */
-extern DECLSPEC int SDLCALL SDL_WinRTRunApp(int (*mainFunction)(int, char **), void * xamlBackgroundPanel);
+extern DECLSPEC int SDLCALL SDL_WinRTRunApp(int (*mainFunction)(int, char **), void * reserved);
 
 #endif /* __WINRT__ */