decoder: move conform function to base class

There's no reason the Conform() function can't live in the base class since its
functionality isn't necessarily specific to FFmpeg.

app/codec/oiio/oiiodecoder.cpp

@@ -187,3 +187,8 @@ bool OIIODecoder::SupportsVideo()
 {
   return true;
 }
+
+QString OIIODecoder::GetIndexFilename()
+{
+  return QString();
+}