dataqueue: Make thread safe.

Each data queue gets its own mutex and each function obtains it.

Fixes #7390.

(cherry picked from commit 8b9a938413)

src/SDL_dataqueue.h

@@ -33,6 +33,7 @@ int SDL_WriteToDataQueue(SDL_DataQueue *queue, const void *data, const size_t le
 size_t SDL_ReadFromDataQueue(SDL_DataQueue *queue, void *buf, const size_t len);
 size_t SDL_PeekIntoDataQueue(SDL_DataQueue *queue, void *buf, const size_t len);
 size_t SDL_CountDataQueue(SDL_DataQueue *queue);
+SDL_mutex *SDL_GetDataQueueMutex(SDL_DataQueue *queue);  /* don't destroy this, obviously. */
 
 #endif /* SDL_dataqueue_h_ */