Perform a resampling and down-mix if required. We down-mix only if encoder is mono and input is stereo. In case of dual-streaming, both encoders has to be mono for down-mix to take place. |*ptr_out| will point to the pre-processed audio-frame. If no pre-processing is required, |*ptr_out| points to |in_frame|. TODO(yujo): Make this more efficient for muted frames.
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