Fix SEEK_END usage in seek implementations.
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.
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