avformat/avformat: Constify AVFormatContext.*_codec pointers

This also allows to exclusively use pointers to const AVCodec in
fftools/ffmpeg_opt.c.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

doc/APIchanges

@@ -14,6 +14,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2021-04-27 - xxxxxxxxxx - lavf yyyyyyyyy - avformat.h
+  Constified AVFormatContext.*_codec.
+
 2021-04-27 - xxxxxxxxxx - lavf yyyyyyyyy - avformat.h
   Constified the pointers to AVInputFormats and AVOutputFormats
   in AVFormatContext, avformat_alloc_output_context2(),

fftools/ffmpeg_opt.c

@@ -737,11 +737,11 @@ static int opt_recording_timestamp(void *optctx, const char *opt, const char *ar
     return 0;
 }
 
-static AVCodec *find_codec_or_die(const char *name, enum AVMediaType type, int encoder)
+static const AVCodec *find_codec_or_die(const char *name, enum AVMediaType type, int encoder)
 {
     const AVCodecDescriptor *desc;
     const char *codec_string = encoder ? "encoder" : "decoder";
-    AVCodec *codec;
+    const AVCodec *codec;
 
     codec = encoder ?
         avcodec_find_encoder_by_name(name) :

libavformat/avformat.h

@@ -1598,7 +1598,7 @@ typedef struct AVFormatContext {
      * the same codec_id.
      * Demuxing: Set by user
      */
-    AVCodec *video_codec;
+    const AVCodec *video_codec;
 
     /**
      * Forced audio codec.
@@ -1606,7 +1606,7 @@ typedef struct AVFormatContext {
      * the same codec_id.
      * Demuxing: Set by user
      */
-    AVCodec *audio_codec;
+    const AVCodec *audio_codec;
 
     /**
      * Forced subtitle codec.
@@ -1614,7 +1614,7 @@ typedef struct AVFormatContext {
      * the same codec_id.
      * Demuxing: Set by user
      */
-    AVCodec *subtitle_codec;
+    const AVCodec *subtitle_codec;
 
     /**
      * Forced data codec.
@@ -1622,7 +1622,7 @@ typedef struct AVFormatContext {
      * the same codec_id.
      * Demuxing: Set by user
      */
-    AVCodec *data_codec;
+    const AVCodec *data_codec;
 
     /**
      * Number of bytes to be written as padding in a metadata header.