fix volume handling on Windows

volume handling for QAudioOutput is broken currently as it fallbacks to
the default implementation.
Need to cache the volume, as hWaveOut is invalid while the output is not
in active state.

Task-number: QTBUG-25454
Change-Id: I2adb28a5e6719a5d3c360553abd70af46b646080
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimedia/audio/qaudiooutput_win32_p.cpp

@@ -117,6 +117,7 @@ QAudioOutputPrivate::QAudioOutputPrivate(const QByteArray &device)
     audioSource = 0;
     pullMode = true;
     finished = false;
+    volumeCache = (qreal)1.;
 }
 
 QAudioOutputPrivate::~QAudioOutputPrivate()
@@ -380,6 +381,8 @@ bool QAudioOutputPrivate::open()
     timeStampOpened.restart();
     elapsedTimeOffset = 0;
 
+    setVolume(volumeCache);
+
     errorState = QAudio::NoError;
     if(pullMode) {
         deviceState = QAudio::ActiveState;
@@ -667,6 +670,25 @@ QAudio::State QAudioOutputPrivate::state() const
     return deviceState;
 }
 
+void QAudioOutputPrivate::setVolume(qreal v)
+{
+    const qreal normalizedVolume = qBound(qreal(0.0), v, qreal(1.0));
+    if (deviceState != QAudio::ActiveState) {
+        volumeCache = normalizedVolume;
+        return;
+    }
+    const qint16 scaled = normalizedVolume * 0xFFFF;
+    DWORD vol = MAKELONG(scaled, scaled);
+    MMRESULT res = waveOutSetVolume(hWaveOut, vol);
+    if (res == MMSYSERR_NOERROR)
+        volumeCache = normalizedVolume;
+}
+
+qreal QAudioOutputPrivate::volume() const
+{
+    return volumeCache;
+}
+
 void QAudioOutputPrivate::reset()
 {
     close();

src/multimedia/audio/qaudiooutput_win32_p.h

@@ -112,6 +112,8 @@ public:
     qint64 elapsedUSecs() const;
     QAudio::Error error() const;
     QAudio::State state() const;
+    void setVolume(qreal);
+    qreal volume() const;
 
     QIODevice* audioSource;
     QAudioFormat settings;
@@ -134,6 +136,7 @@ private:
     qint64 totalTimeValue;
     bool pullMode;
     int intervalTime;
+    qreal volumeCache;
     static void QT_WIN_CALLBACK waveOutProc( HWAVEOUT hWaveOut, UINT uMsg,
             DWORD dwInstance, DWORD dwParam1, DWORD dwParam2 );