Added a single source of SDL object IDs

This ensures that we don't accidentally interpret an ID from one system as an ID in another system.
Audio device IDs are not covered here, since they have a unique numbering system.

src/SDL_internal.h

@@ -197,6 +197,7 @@
 extern "C" {
 #endif
 
+extern DECLSPEC Uint32 SDLCALL SDL_GetNextObjectID(void);
 extern DECLSPEC int SDLCALL SDL_WaitSemaphoreTimeoutNS(SDL_Semaphore *sem, Sint64 timeoutNS);
 extern DECLSPEC int SDLCALL SDL_WaitConditionTimeoutNS(SDL_Condition *cond, SDL_Mutex *mutex, Sint64 timeoutNS);
 extern DECLSPEC int SDLCALL SDL_WaitEventTimeoutNS(SDL_Event *event, Sint64 timeoutNS);