OggOpusWriter: Incorrect duration when sample rate is not 48kHz.

[ijc8]

For supported sample rates other than 48k (i.e. 8k, 12k, 16k, 24k), OggOpusWriter generates an incorrectly reported duration, which causes bizarre behaviors depending on the audio player.

To reproduce:

• Convert examples/left-right-demo-5s.wav to 12k (e.g. sox left-right-demo-5s.wav -r 12k 12k.wav; mv 12k.wav left-right-demo-5s.wav)
• Run examples/03-write-ogg-opus.py
• This generates left-right-demo-5s.wav.opus with an incorrect duration (ffmpeg reports 1.26s; opusinfo reports 2.149s), which will confuse various tools and audio players (e.g. VLC and Totem cut off early; pyogg throws an exception on open.)

Initially, I thought this was because OggOpusWriter always writes a sample rate of 0 (unspecified) in the Opus header. However, specifying the original sample rate in the header does not correct this duration issue:

import pyogg

sr = 12000

encoder = pyogg.OpusBufferedEncoder()
encoder.set_application("audio")
encoder.set_sampling_frequency(sr)
encoder.set_channels(1)
encoder.set_frame_size(20)

writer = pyogg.OggOpusWriter("should_be_1s.opus", encoder)
# HACK: Get OggOpusWriter to specify the sample rate in the header.
pyogg.OggOpusWriter._make_identification_header.__defaults__ = (sr,)
bytes_per_sample = 2
samples = bytearray(sr * bytes_per_sample)
writer.write(samples)
writer.close()

Running opusinfo on should_be_1s.opus correctly reports "Original sample rate: 12000Hz", but still reports a playback length of .25 seconds.
Any idea what's going wrong?