Add a windowID field to SDL_TouchFingerEvent (bug #4331).

This is unimplemented on some platforms and will cause compile errors when building those platform backends for now.

include/SDL_events.h

@@ -442,6 +442,7 @@ typedef struct SDL_TouchFingerEvent
     float dx;           /**< Normalized in the range -1...1 */
     float dy;           /**< Normalized in the range -1...1 */
     float pressure;     /**< Normalized in the range 0...1 */
+    Uint32 windowID;    /**< The window underneath the finger, if any */
 } SDL_TouchFingerEvent;