Support Read Sample Rates other than 48k

[zacharyneveu]

I have a use case where I am reading many opus files which need to be at a sample rate other than 48k. Currently I'm using opusfile to read at 48k, then libsamplerate to re-sample, but this is taking a significant amount of compute time. It would be very convenient to be able to read from opus files directly at my target sample rate and avoid resampling to save compute. Would anyone be able to comment on how much work would be involved to support this? I have some time to devote to this, and could make a PR if this is feasible.

[rillian]

libopus supports efficient decoding at fixed fractions of 48 kHz, so if you want 8, 12, 16, or 24 kHz, that could probably be done. You need to replace the hard-coded sample rate with an enum, and then add a configuration switch to chose one of the lower sample rates.

For any other sample rates, like 44.1 kHz, you still need to run a resampler, so best to look at ways to do that more efficiently.