began audio conform code in ffmpegdecoder

For accuracy, whenever we need to perform an audio resample, we need to do it
in advance. This is in a process called "conforming" and this commit introduces
the framework by which the decoder can automatically conform an audio stream
to arbitrary parameters for accurate rendering.

app/decoder/waveinput.cpp

@@ -124,7 +124,8 @@ bool WaveInput::open()
     return false;
   }
 
-  data_position_ = file_.pos() + 4;
+  data_stream >> data_size_;
+  data_position_ = file_.pos();
 
   return true;
 }
@@ -166,6 +167,11 @@ void WaveInput::close()
   }
 }
 
+int WaveInput::sample_count()
+{
+  return params_.bytes_to_samples(static_cast<int>(data_size_));
+}
+
 bool WaveInput::find_str(QFile *f, const char *str)
 {
   qint64 pos = f->pos();