Allow specify other channel counts for PortAudio playback

The default channel configuration is stereo. The user is responsible for
ensuring the requested channels are compatible with the device configuration
since PortAudio provides no way to check.

alc/backends/portaudio.cpp

@@ -89,6 +89,8 @@ struct PortPlayback final : public BackendBase {
 
     PaStream *mStream{nullptr};
     PaStreamParameters mParams{};
+    DevFmtChannels mChannelConfig{};
+    uint mAmbiOrder{};
     uint mUpdateSize{0u};
 };
 
@@ -125,7 +127,9 @@ void PortPlayback::open(std::string_view name)
     params.suggestedLatency = mDevice->BufferSize / static_cast<double>(mDevice->Frequency);
     params.hostApiSpecificStreamInfo = nullptr;
 
-    params.channelCount = ((mDevice->FmtChans == DevFmtMono) ? 1 : 2);
+    mChannelConfig = mDevice->FmtChans;
+    mAmbiOrder = mDevice->mAmbiOrder;
+    params.channelCount = static_cast<int>(mDevice->channelsFromFmt());
 
     switch(mDevice->FmtType)
     {
@@ -179,7 +183,18 @@ bool PortPlayback::reset()
 {
     const PaStreamInfo *streamInfo{Pa_GetStreamInfo(mStream)};
     mDevice->Frequency = static_cast<uint>(streamInfo->sampleRate);
+    mDevice->FmtChans = mChannelConfig;
+    mDevice->mAmbiOrder = mAmbiOrder;
     mDevice->UpdateSize = mUpdateSize;
+    mDevice->BufferSize = mUpdateSize * 2;
+    if(streamInfo->outputLatency > 0.0f)
+    {
+        const double sampleLatency{streamInfo->outputLatency * streamInfo->sampleRate};
+        TRACE("Reported stream latency: %f sec (%f samples)\n", streamInfo->outputLatency,
+            sampleLatency);
+        mDevice->BufferSize = static_cast<uint>(std::clamp(sampleLatency,
+            double(mDevice->BufferSize), double{std::numeric_limits<int>::max()}));
+    }
 
     if(mParams.sampleFormat == paInt8)
         mDevice->FmtType = DevFmtByte;
@@ -197,15 +212,6 @@ bool PortPlayback::reset()
         return false;
     }
 
-    if(mParams.channelCount >= 2)
-        mDevice->FmtChans = DevFmtStereo;
-    else if(mParams.channelCount == 1)
-        mDevice->FmtChans = DevFmtMono;
-    else
-    {
-        ERR("Unexpected channel count: %u\n", mParams.channelCount);
-        return false;
-    }
     setDefaultChannelOrder();
 
     return true;