SDL_WarpMouseGlobal() should return non-void.

There are platforms it isn't implemented on (and currently can't be
implemented on!), and there's currently no way for an app to know this.

This shouldn't break ABI on apps that moved to a revision between 2.0.3 and
2.0.4.

include/SDL_mouse.h

@@ -137,10 +137,11 @@ extern DECLSPEC void SDLCALL SDL_WarpMouseInWindow(SDL_Window * window,
  *
  *  \param x The x coordinate
  *  \param y The y coordinate
+ *  \return 0 on success, -1 on error (usually: unsupported by a platform).
  *
  *  \note This function generates a mouse motion event
  */
-extern DECLSPEC void SDLCALL SDL_WarpMouseGlobal(int x, int y);
+extern DECLSPEC int SDLCALL SDL_WarpMouseGlobal(int x, int y);
 
 /**
  *  \brief Set relative mouse mode.