add PortAudio backend - PLEASE READ

PLEASE DO:

```
git submodule update --init --recursive
```

AFTER PULLING THIS COMMIT.

src/audio/pa.cpp

@@ -0,0 +1,222 @@
+/**
+ * Furnace Tracker - multi-system chiptune tracker
+ * Copyright (C) 2021-2023 tildearrow and contributors
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <string.h>
+#include <vector>
+#include "../ta-log.h"
+#include "pa.h"
+
+int taPAProcess(const void* in, void* out, unsigned long nframes, const PaStreamCallbackTimeInfo* timeInfo, PaStreamCallbackFlags flags, void* inst) {
+  TAAudioPA* instance=(TAAudioPA*)inst;
+  return instance->onProcess(in,out,nframes,timeInfo,flags);
+}
+
+int TAAudioPA::onProcess(const void* in, void* out, unsigned long nframes, const PaStreamCallbackTimeInfo* timeInfo, PaStreamCallbackFlags flags) {
+  for (int i=0; i<desc.inChans; i++) {
+    if (nframes>desc.bufsize) {
+      delete[] inBufs[i];
+      inBufs[i]=new float[nframes];
+    }
+  }
+  for (int i=0; i<desc.outChans; i++) {
+    if (nframes>desc.bufsize) {
+      delete[] outBufs[i];
+      outBufs[i]=new float[nframes];
+    }
+  }
+  if (nframes!=desc.bufsize) {
+    desc.bufsize=nframes;
+  }
+
+  if (audioProcCallback!=NULL) {
+    if (midiIn!=NULL) midiIn->gather();
+    audioProcCallback(audioProcCallbackUser,inBufs,outBufs,desc.inChans,desc.outChans,desc.bufsize);
+  }
+  float* fbuf=(float*)out;
+  for (size_t j=0; j<desc.bufsize; j++) {
+    for (size_t i=0; i<desc.outChans; i++) {
+      fbuf[j*desc.outChans+i]=outBufs[i][j];
+    }
+  }
+  return 0;
+}
+
+void* TAAudioPA::getContext() {
+  return (void*)&ac;
+}
+
+bool TAAudioPA::quit() {
+  if (!initialized) return false;
+
+  Pa_CloseStream(ac);
+
+  if (running) {
+    running=false;
+  }
+
+  for (int i=0; i<desc.outChans; i++) {
+    delete[] outBufs[i];
+  }
+
+  delete[] outBufs;
+  
+  initialized=false;
+  return true;
+}
+
+bool TAAudioPA::setRun(bool run) {
+  if (!initialized) return false;
+  if (running==run) return running;
+  PaError status;
+  if (run) {
+    status=Pa_StartStream(ac);
+  } else {
+    status=Pa_StopStream(ac);
+  }
+  if (status!=paNoError) {
+    logW("error while setting run status: %s",Pa_GetErrorText(status));
+    return running;
+  }
+  running=run;
+  return running;
+}
+
+std::vector<String> TAAudioPA::listAudioDevices() {
+  std::vector<String> ret;
+  if (!audioSysStarted) {
+    PaError status=Pa_Initialize();
+    if (status!=paNoError) {
+      logE("could not initialize PortAudio to list audio devices");
+      return ret;
+    } else {
+      audioSysStarted=true;
+    }
+  }
+
+  int count=Pa_GetDeviceCount();
+  if (count<0) return ret;
+
+  for (int i=0; i<count; i++) {
+    const PaDeviceInfo* devInfo=Pa_GetDeviceInfo(i);
+    if (devInfo==NULL) continue;
+    if (devInfo->maxOutputChannels<1) continue;
+
+    if (devInfo->name!=NULL) {
+      ret.push_back(String(devInfo->name));
+    }
+  }
+
+  return ret;
+}
+
+bool TAAudioPA::init(TAAudioDesc& request, TAAudioDesc& response) {
+  if (initialized) {
+    logE("audio already initialized");
+    return false;
+  }
+  PaError status;
+
+  if (!audioSysStarted) {
+    status=Pa_Initialize();
+    if (status!=paNoError) {
+      logE("could not initialize PortAudio");
+      return false;
+    } else {
+      audioSysStarted=true;
+    }
+  }
+
+  desc=request;
+  desc.outFormat=TA_AUDIO_FORMAT_F32;
+
+  const PaDeviceInfo* devInfo=NULL;
+  int outDeviceID=0;
+
+  if (desc.deviceName.empty()) {
+    outDeviceID=Pa_GetDefaultOutputDevice();
+    devInfo=Pa_GetDeviceInfo(outDeviceID);
+  } else {
+    int count=Pa_GetDeviceCount();
+    bool found=false;
+    if (count<0) {
+      logE("audio device not found");
+      return false;
+    }
+
+    for (int i=0; i<count; i++) {
+      devInfo=Pa_GetDeviceInfo(i);
+      if (devInfo==NULL) continue;
+      if (devInfo->maxOutputChannels<1) continue;
+
+      if (devInfo->name!=NULL) {
+        if (strcmp(devInfo->name,desc.deviceName.c_str())==0) {
+          outDeviceID=i;
+          found=true;
+          break;
+        }
+      }
+    }
+    if (!found) {
+      logE("audio device not found");
+      return false;
+    }
+  }
+
+  // check output channels and sample rate
+  if (devInfo!=NULL) {
+    if (desc.outChans>devInfo->maxOutputChannels) desc.outChans=devInfo->maxOutputChannels;
+  }
+
+  PaStreamParameters outParams;
+  outParams.device=outDeviceID;
+  outParams.channelCount=desc.outChans;
+  outParams.sampleFormat=paFloat32;
+  outParams.suggestedLatency=(double)(desc.bufsize*desc.fragments)/desc.rate;
+  outParams.hostApiSpecificStreamInfo=NULL;
+
+  logV("opening audio device...");
+  status=Pa_OpenStream(
+    &ac,
+    NULL,
+    &outParams,
+    desc.rate,
+    0,
+    paClipOff|paDitherOff,
+    taPAProcess,
+    this
+  );
+  if (status!=paNoError) {
+    logE("could not open audio device: %s",Pa_GetErrorText(status));
+    return false;
+  }
+
+  desc.deviceName=devInfo->name;
+  desc.inChans=0;
+
+  if (desc.outChans>0) {
+    outBufs=new float*[desc.outChans];
+    for (int i=0; i<desc.outChans; i++) {
+      outBufs[i]=new float[desc.bufsize];
+    }
+  }
+
+  response=desc;
+  initialized=true;
+  return true;
+}

src/audio/pa.h

@@ -0,0 +1,38 @@
+/**
+ * Furnace Tracker - multi-system chiptune tracker
+ * Copyright (C) 2021-2023 tildearrow and contributors
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "taAudio.h"
+#include <portaudio.h>
+
+class TAAudioPA: public TAAudio {
+  PaStream* ac;
+  bool audioSysStarted;
+
+  public:
+    int onProcess(const void* in, void* out, unsigned long nframes, const PaStreamCallbackTimeInfo* timeInfo, PaStreamCallbackFlags flags);
+
+    void* getContext();
+    bool quit();
+    bool setRun(bool run);
+    std::vector<String> listAudioDevices();
+    bool init(TAAudioDesc& request, TAAudioDesc& response);
+    TAAudioPA():
+      ac(NULL),
+      audioSysStarted(false) {}
+};