Harmonize op_raw_total() and op_raw_seek()
op_raw_seek() will fail if you try to seek to a byte position beyond the end of the file. However, the "end" is defined in terms of _of->end, which is specifically the end of the last Ogg page found in the underlying source (excluding any trailing non-Ogg data). op_raw_total(_of,-1) returns the total size of the stream by using _of->end, but it was also subtracting the offset of the first Opus page in the first link. Since there might have been other Ogg streams concurrently multiplexed with the first Opus stream whose BOS pages appear first, or there might simply be non-Ogg junk at the start, that left the caller with no way to determine the valid range of byte offsets that could be passed to op_raw_seek(). Instead, make op_raw_total() pretend the first link starts at offset 0, and explicitly document that it's what defines the range of valid values to op_raw_seek(). This is how our own seeking_example.c was using it, anyway.
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