Allow for tx periods shorter than the audio buffer size

Audio will not start until at least one buffer full is achieved and as
we use a large target latency of 2s to minimize CPU usage and glitches
we must pad with silence when  the QAudioOutput pulls buffers from the
Modulator i/o device. This is  all necessary with pulseaudio using the
underlying o/s ALSA device, i.e. on Linux.

Modulator.cpp

@@ -319,6 +319,12 @@ qint64 Modulator::readData (char * data, qint64 maxSize)
         // done for this chunk - continue on next call
 //        qint64 ms1=QDateTime::currentMSecsSinceEpoch() - m_ms0;
 //        if(m_ic>=4*144*160) qDebug() << "Modulator finished" << m_ic << 0.001*ms1;
+
+        while (samples != end)  // pad block with silence
+          {
+            samples = load (0, samples);
+            ++framesGenerated;
+          }
         return framesGenerated * bytesPerFrame ();
       }
       // fall through

soundout.cpp

@@ -101,7 +101,7 @@ void SoundOutput::restart (QIODevice * source)
   m_stream->setBufferSize (m_stream->format().bytesForDuration((m_msBuffered ? m_msBuffered : MS_BUFFERED) * 1000));
   //  qDebug() << "B" << m_stream->bufferSize() <<
   //  m_stream->periodSize() << m_stream->notifyInterval();
-
+  m_stream->setCategory ("production");
   m_stream->start (source);
 }