apetag: fix flag value to signal footer presence

According to the spec[1], a value of 0 means the footer is present and a value
of 1 means it's absent, the exact opposite of header presence flag where 1
means present and 0 absent.
The reason for this is compatibility with APEv1 tags, where there's no header,
footer presence was mandatory for all files, and the flags field was a zeroed
reserved field.

[1] http://wiki.hydrogenaud.io/index.php?title=Ape_Tags_FlagsSigned-off-by: James Almer <jamrial@gmail.com>

CC: libav-stable@libav.org
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/apetag.c

@@ -32,7 +32,7 @@
 #define APE_TAG_VERSION               2000
 #define APE_TAG_FOOTER_BYTES          32
 #define APE_TAG_FLAG_CONTAINS_HEADER  (1 << 31)
-#define APE_TAG_FLAG_CONTAINS_FOOTER  (1 << 30)
+#define APE_TAG_FLAG_LACKS_FOOTER     (1 << 30)
 #define APE_TAG_FLAG_IS_HEADER        (1 << 29)
 #define APE_TAG_FLAG_IS_BINARY        (1 << 1)
 
@@ -194,8 +194,7 @@ int ff_ape_write_tag(AVFormatContext *s)
     avio_wl32(s->pb, 0);                // reserve space for tag count
 
     // flags
-    avio_wl32(s->pb, APE_TAG_FLAG_CONTAINS_HEADER | APE_TAG_FLAG_CONTAINS_FOOTER |
-                     APE_TAG_FLAG_IS_HEADER);
+    avio_wl32(s->pb, APE_TAG_FLAG_CONTAINS_HEADER | APE_TAG_FLAG_IS_HEADER);
     ffio_fill(s->pb, 0, 8);             // reserved
 
     while ((e = av_dict_get(s->metadata, "", e, AV_DICT_IGNORE_SUFFIX))) {
@@ -217,7 +216,7 @@ int ff_ape_write_tag(AVFormatContext *s)
     avio_wl32(s->pb, count);            // tag count
 
     // flags
-    avio_wl32(s->pb, APE_TAG_FLAG_CONTAINS_HEADER | APE_TAG_FLAG_CONTAINS_FOOTER);
+    avio_wl32(s->pb, APE_TAG_FLAG_CONTAINS_HEADER);
     ffio_fill(s->pb, 0, 8);             // reserved
 
     // update values in the header