avcodec/mpeg12enc: Simplify check for A53 closed captions

The a53_cc option is only useful and meaningful for MPEG-2,
yet it was accidentally added for all mpegvideo-based encoders.
This means that it is possible for a53_cc to be set for other
encoders as well.
This commit changes this and reroutes a53_cc to the dummy field
in MpegEncContext for all codecs for which it is not supported.
This allows to avoid a check for the current codec in mpeg12enc.c.

Also add a compile-time check for whether the MPEG-2 encoder is
available while at it.

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

libavcodec/mpeg12enc.c

@@ -551,7 +551,7 @@ void ff_mpeg1_encode_picture_header(MpegEncContext *s, int picture_number)
         }
     }
 
-    if (s->codec_id == AV_CODEC_ID_MPEG2VIDEO && s->a53_cc) {
+    if (CONFIG_MPEG2VIDEO_ENCODER && s->a53_cc) {
         side_data = av_frame_get_side_data(s->current_picture_ptr->f,
             AV_FRAME_DATA_A53_CC);
         if (side_data) {

libavcodec/mpegvideo.h

@@ -662,7 +662,7 @@ FF_MPV_OPT_CMP_FUNC, \
 #define FF_MPV_DEPRECATED_MPEG_QUANT_OPT \
     { "mpeg_quant", "Deprecated, does nothing", FF_MPV_OFFSET(mpeg_quant), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, 0, FF_MPV_OPT_FLAGS | AV_OPT_FLAG_DEPRECATED },
 #define FF_MPV_DEPRECATED_A53_CC_OPT \
-    { "a53cc",      "Deprecated, does nothing", FF_MPV_OFFSET(a53_cc),     AV_OPT_TYPE_BOOL, { .i64 = 1 }, 0, 1, FF_MPV_OPT_FLAGS | AV_OPT_FLAG_DEPRECATED },
+    { "a53cc",      "Deprecated, does nothing", FF_MPV_OFFSET(dummy),      AV_OPT_TYPE_BOOL, { .i64 = 1 }, 0, 1, FF_MPV_OPT_FLAGS | AV_OPT_FLAG_DEPRECATED },
 #define FF_MPV_DEPRECATED_MATRIX_OPT \
    { "force_duplicated_matrix", "Deprecated, does nothing", FF_MPV_OFFSET(dummy), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0, 1, FF_MPV_OPT_FLAGS | AV_OPT_FLAG_DEPRECATED },
 #define FF_MPV_DEPRECATED_BFRAME_OPTS \