When seeking using SEEK_END, the win32-specific implementation of
 op_fseek() would add the offset to the file size to get the
 absolute seek position.
However, op_mem_seek() and op_http_stream_seek() would subtract the
 offset instead.
The documentation of fseek() is not at all clear which behavior is
 correct, but I believe that the op_fseek() behavior is.

This inconsistency didn't matter for opusfile in practice, because
 we only ever use SEEK_END with an offset of 0.
However, the user can also open files with our API and use the
 resulting callbacks for their own purposes, so it would be good to
 be consistent for them.

Thanks to a person who did not wish to be credited for the report.