ffmpeg: allow setting attached_pic disposition

This is used to signal that image should be stored in metadata
as cover image.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

fftools/ffmpeg.c

@@ -3561,6 +3561,7 @@ static int init_output_stream(OutputStream *ost, char *error, int error_len)
             { "hearing_impaired"    , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_HEARING_IMPAIRED  },    .unit = "flags" },
             { "visual_impaired"     , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_VISUAL_IMPAIRED   },    .unit = "flags" },
             { "clean_effects"       , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_CLEAN_EFFECTS     },    .unit = "flags" },
+            { "attached_pic"        , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_ATTACHED_PIC      },    .unit = "flags" },
             { "captions"            , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_CAPTIONS          },    .unit = "flags" },
             { "descriptions"        , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_DESCRIPTIONS      },    .unit = "flags" },
             { "dependent"           , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_DEPENDENT         },    .unit = "flags" },