Added SDL_AddAtomicU32()

Fixes https://github.com/libsdl-org/SDL/issues/13496

include/SDL3/SDL_atomic.h

@@ -596,6 +596,24 @@ extern SDL_DECLSPEC Uint32 SDLCALL SDL_SetAtomicU32(SDL_AtomicU32 *a, Uint32 v);
  */
 extern SDL_DECLSPEC Uint32 SDLCALL SDL_GetAtomicU32(SDL_AtomicU32 *a);
 
+/**
+ * Add to an atomic variable.
+ *
+ * This function also acts as a full memory barrier.
+ *
+ * ***Note: If you don't know what this function is for, you shouldn't use
+ * it!***
+ *
+ * \param a a pointer to an SDL_AtomicU32 variable to be modified.
+ * \param v the desired value to add or subtract.
+ * \returns the previous value of the atomic variable.
+ *
+ * \threadsafety It is safe to call this function from any thread.
+ *
+ * \since This function is available since SDL 3.2.0.
+ */
+extern SDL_DECLSPEC Uint32 SDLCALL SDL_AddAtomicU32(SDL_AtomicU32 *a, int v);
+
 /**
  * Set a pointer to a new value if it is currently an old value.
  *