the SDL_IOStream to read all available data from.
a pointer filled in with the number of bytes read, may be NULL.
if true, calls SDL_CloseIO() on src before returning, even in the case of an error.
the data or NULL on failure; call SDL_GetError() for more information.
Threadsafety: This function is not thread safe.
Load all the data from an SDL data stream.
The data is allocated with a zero byte at the end (null terminated) for convenience. This extra byte is not included in the value reported via datasize.
The data should be freed with SDL_free().