fftools/ffmpeg_enc: constify the frame passed to enc_open()

fftools/ffmpeg.h

@@ -817,7 +817,7 @@ int dec_packet(InputStream *ist, const AVPacket *pkt, int no_eof);
 int enc_alloc(Encoder **penc, const AVCodec *codec);
 void enc_free(Encoder **penc);
 
-int enc_open(OutputStream *ost, AVFrame *frame);
+int enc_open(OutputStream *ost, const AVFrame *frame);
 int enc_subtitle(OutputFile *of, OutputStream *ost, const AVSubtitle *sub);
 int enc_frame(OutputStream *ost, AVFrame *frame);
 int enc_flush(void);

fftools/ffmpeg_enc.c

@@ -165,7 +165,7 @@ static int set_encoder_id(OutputFile *of, OutputStream *ost)
     return 0;
 }
 
-int enc_open(OutputStream *ost, AVFrame *frame)
+int enc_open(OutputStream *ost, const AVFrame *frame)
 {
     InputStream *ist = ost->ist;
     Encoder              *e = ost->enc;
@@ -183,9 +183,8 @@ int enc_open(OutputStream *ost, AVFrame *frame)
     av_assert0(frame || (enc->type != AVMEDIA_TYPE_VIDEO && enc->type != AVMEDIA_TYPE_AUDIO));
 
     if (frame) {
-        fd = frame_data(frame);
-        if (!fd)
-            return AVERROR(ENOMEM);
+        av_assert0(frame->opaque_ref);
+        fd = (FrameData*)frame->opaque_ref->data;
     }
 
     ret = set_encoder_id(output_files[ost->file_index], ost);