decoder_api: pass "seekable" flag to decoder_initialized()

Don't pass the "seekable" flag with every decoder_data() invocation.
Since that flag won't change within the file, it is enough to pass it
to decoder_initialized() once per file.

src/decoder/oggflac_plugin.c

@@ -303,7 +303,8 @@ oggflac_decode(struct decoder * mpd_decoder, struct input_stream *inStream)
 		goto fail;
 	}
 
-	decoder_initialized(mpd_decoder, &data.audio_format, data.total_time);
+	decoder_initialized(mpd_decoder, &data.audio_format,
+			    inStream->seekable, data.total_time);
 
 	while (true) {
 		OggFLAC__seekable_stream_decoder_process_single(decoder);