the SDL_IOStream to write all data to.
the data to be written. If datasize is 0, may be NULL or a invalid pointer.
the number of bytes to be written.
if true, calls SDL_CloseIO() on src before returning, even in the case of an error.
true on success or false on failure; call SDL_GetError() for more information.
Threadsafety: This function is not thread safe.
Save all the data into an SDL data stream.