Errors when recording from type multi ALSA PCM device

[carlocaione]

Describe the bug

jack2 is mixing channels and giving often errors when recording from a type multi PCM ALSA device (seen in #599 as well).

Environment

• JACK Version: Reproduced on current master
• Operating System: Linux

Steps To Reproduce

Create a new PCM device to merge 2 16-channels inputs using a type multi ALSA plugin. I'm using this configuration: https://pastebin.com/raw/ELNFZ4dt

To start the daemon:

# JACK_NO_AUDIO_RESERVATION=1 jackd -R -P 30 -d alsa -d merge -r 48000 -s -n 3 -p 128 -i 32 -o 32

To record:

# jack_rec -f out.wav -d 4 system:capture_1 system:capture_2 system:capture_3 system:capture_4 system:capture_5
                           system:capture_6 system:capture_7 system:capture_8 system:capture_9 system:capture_10
                           system:capture_11 system:capture_12 system:capture_13 system:capture_14 system:capture_15
                           system:capture_16 system:capture_17 system:capture_18 system:capture_19 system:capture_20
                           system:capture_21 system:capture_22 system:capture_23 system:capture_24 system:capture_25
                           system:capture_26 system:capture_27 system:capture_28 system:capture_29 system:capture_30
                           system:capture_31 system:capture_32

or:

# jack_capture -b 32 -c 32 -d 3 -p system:capture* -f wav o.wav

Expected vs. actual behavior

I'm expected to see a 32-channels output wav file with interleaved channels coming from the two PCM slaves. What I get is a wave file where the channels are randomly allocated and often the jackd daemon is complaining as described in #599