avcodec/mpegvideo: Constify src of ff_update_duplicate_context()

Also do the same for update_duplicate_context_after_me()
in mpegvideo_enc.c.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mpegvideo.c

@@ -469,7 +469,7 @@ static void backup_duplicate_context(MpegEncContext *bak, MpegEncContext *src)
 #undef COPY
 }
 
-int ff_update_duplicate_context(MpegEncContext *dst, MpegEncContext *src)
+int ff_update_duplicate_context(MpegEncContext *dst, const MpegEncContext *src)
 {
     MpegEncContext bak;
     int i, ret;

libavcodec/mpegvideo.h

@@ -589,7 +589,7 @@ void ff_mpv_reconstruct_mb(MpegEncContext *s, int16_t block[12][64]);
 
 void ff_clean_intra_table_entries(MpegEncContext *s);
 
-int ff_update_duplicate_context(MpegEncContext *dst, MpegEncContext *src);
+int ff_update_duplicate_context(MpegEncContext *dst, const MpegEncContext *src);
 void ff_set_qscale(MpegEncContext * s, int qscale);
 
 void ff_mpv_idct_init(MpegEncContext *s);

libavcodec/mpegvideo_enc.c

@@ -250,7 +250,7 @@ void ff_init_qscale_tab(MpegEncContext *s)
 }
 
 static void update_duplicate_context_after_me(MpegEncContext *dst,
-                                              MpegEncContext *src)
+                                              const MpegEncContext *src)
 {
 #define COPY(a) dst->a= src->a
     COPY(pict_type);