Implement the Channel Mapping API of ALSA

[jcucurull]

Feature request

Implement the Channel Mapping API of ALSA (https://www.kernel.org/doc/html/v4.16/sound/designs/channel-mapping-api.html), via libasound (see e.g. https://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m.html#ga60ee7d2c2555e21dbc844a1b73839085), to support the customization of the audio channel mappings in systems that support more than two channels, e.g. 5.1 or 7.1 audio systems.

Currently, the mapping used in MPD is the default of ALSA (see /usr/share/alsa/pcm/surround71.conf) and it is hardcoded:

MPD/src/pcm/Order.cxx

Line 62 in 381215f

TwoPointers<V> &ToAlsa51() noexcept {

However, not all the sound systems follow this mapping (e.g. in my case, using the HDMI in RP3 connected to an AVR the mapping is not correct, see discussion here #2060).