PortAudio sink plays little-endian PCM as native-endian on big-endian hosts #78

Found while auditing for #70 (fixed in #77).

The problem

The PCM we hand every sink is little-endian. src/portaudio_sink.cpp opens its stream with paInt16, paInt24 or paInt32 (PROBE_FORMATS, chosen by pa_format_for()), and PortAudio defines those as native-endian. The callback copies our bytes straight into PortAudio's buffer, so on a big-endian host PortAudio reads every 16-, 24- and 32-bit sample with its bytes reversed and plays noise, at any volume including unity.

The doc comment on pa_format_for() says it maps onto "the interleaved little-endian PCM format PortAudio wants", which is only true on a little-endian host.

8-bit is unaffected. The other sinks are unaffected: ALSA, PulseAudio, PipeWire and CoreAudio all open explicitly little-endian formats.

Status

Read-confirmed only, not reproduced. Unlike #70 it cannot be tested with a standalone file under qemu-mips-static, since it needs PortAudio and a device.

Fix

PortAudio has no little-endian format to ask for, so this needs a byte swap of each sample in the callback on big-endian hosts, after the volume is applied (apply_volume_ramp() expects little-endian). Little-endian hosts should compile to no extra work.

Impact

Big-endian builds using --output portaudio only. ALSA is the usual backend on the OpenWrt MIPS targets that prompted #70, so this is low priority.