seek backward to a key frame for accurate seek

src/AVDemuxer.cpp

@@ -155,7 +155,7 @@ AVDemuxer::AVDemuxer(const QString& fileName, QObject *parent)
     , _file_name(fileName)
     , m_pQAVIO(0)
     , mSeekUnit(SeekByTime)
-    , mSeekTarget(SeekTarget_KeyFrame)
+    , mSeekTarget(SeekTarget_AccurateFrame)
     , mpDict(0)
 {
     mpInterrup = new InterruptHandler(this);
@@ -379,6 +379,9 @@ bool AVDemuxer::seek(qint64 pos)
      * from AV_TIME_BASE units to the stream specific time_base.
      */
     int seek_flag = (backward ? AVSEEK_FLAG_BACKWARD : 0);
+    if (mSeekTarget == SeekTarget_AccurateFrame) {
+        seek_flag = AVSEEK_FLAG_BACKWARD;
+    }
     if (mSeekTarget == SeekTarget_AnyFrame) {
         seek_flag = AVSEEK_FLAG_ANY;
     }