The SDL_AsyncIO to get the size of the data stream from.
the size of the data stream in the SDL_IOStream on success or a negative error code on failure; call SDL_GetError for more information.
Threadsafety: It is safe to call this function from any thread.
Available since SDL 3.2.0.
Use this function to get the size of the data stream in an SDL_AsyncIO.
This call is *not* asynchronous; it assumes that obtaining this info is a non-blocking operation in most reasonable cases.