OPUS_GET_BITRATE always returns default value for opus_multistream_encoder

When I call the OPUS_GET_BITRATE control on a multistream encoder, it always returns the default value, e.g. 144000 for 2 channels at 48 kHz

int32_t bitrate = 64000; // could also be OPUS_BITRATE_MAX

opus_multistream_encoder_ctl(state, OPUS_SET_BITRATE(bitrate));

 // always returns the default...
opus_multistream_encoder_ctl(state, OPUS_GET_BITRATE(&bitrate));

other controls like OPUS_GET_COMPLEXITY or OPUS_GET_SIGNAL work as expected.

Here's the reason:

opus_multistream_encoder has a dedicated field bitrate_bps and OPUS_SET_BITRATE sets it as expected. OPUS_GET_BITRATE, however, internally calls opus_encoder_ctl on the sub-states, which gets the value of user_bitrate_bps. Since the latter is initialized to OPUS_AUTO and never overriden, it would always return the value of the default bitrate.

Not sure what's the best approach to fix this...

Start at opus_multistream_encoder_ctl and trace the OPUS_SET_BITRATE and OPUS_GET_BITRATE paths into opus_encoder_ctl. Compare the multistream encoder's bitrate_bps with the sub-state user_bitrate_bps behavior. Done means the getter reports the bitrate previously set on a multistream encoder, including OPUS_BITRATE_MAX, rather than the default value.