external rate plugin: broken 16-bit stereo stream conversion (interleaved -> non-interleaved)

[ossilator]

this setup:

Plug PCM: Rate conversion PCM (44100, sformat=S16_LE)
Converter: libspeex (external)
Protocol version: 10003
Its setup is:
  stream       : PLAYBACK
  access       : RW_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 2
  rate         : 48000
  exact rate   : 48000 (48000/1)
  msbits       : 16
  buffer_size  : 17832
  period_size  : 4458
  period_time  : 92879
  tstamp_mode  : NONE
  tstamp_type  : MONOTONIC
  period_step  : 1
  avail_min    : 4458
  period_event : 0
  start_threshold  : 17832
  stop_threshold   : 17832
  silence_threshold: 0
  silence_size : 0
  boundary     : 5019261784704417792
Slave: Hardware PCM card 0 'E-MU 0404b PCI [MAEM8852]' device 3 subdevice 0
Its setup is:
  stream       : PLAYBACK
  access       : MMAP_NONINTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 2
  rate         : 44100
  exact rate   : 44100 (44100/1)
  msbits       : 16
  buffer_size  : 16384
  period_size  : 4096
  period_time  : 92879
  tstamp_mode  : NONE
  tstamp_type  : MONOTONIC
  period_step  : 1
  avail_min    : 4096
  period_event : 0
  start_threshold  : 16384
  stop_threshold   : 16384
  silence_threshold: 0
  silence_size : 0
  boundary     : 4611686018427387904
  appl_ptr     : 0
  hw_ptr       : 0

garbles the output rather impressively, though it's still recognizable as the input, more or less.

it happens both when up-sampling and down-sampling. it doesn't happen when not re-sampling.

it doesn't happen when the hardware takes 32-bit samples, so i guess it's somewhere in the optimized s16 paths.

it doesn't happen with mono.

this is not reproducible with the upstream emu10k1 driver, as the multichannel device demands 16 channels, which causes the route conversion plugin to be inserted, which sidesteps the issue. and the regular pcm device accepts any rate.