avformat/id3v2: allow ID3 parsing without AVFormatContext

Add ff_id3v2_read_dict() for parsing without AVFormatContext, but
instead with AVIOContext and AVDictionary.

AVFormatContext is still used for logging, if available.

Chapter parsing is the only non-logging functionality that actually
needs AVFormatContext, and AFAICS it should be modified to write the
data to ID3v2ExtraMeta first, from where it can be implanted to
AVFormatContext by a separate function (like it is done with
read_apic() and ff_id3v2_parse_apic()). That is outside the scope of
this patch, though.
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>

libavformat/id3v2.h

@@ -95,7 +95,21 @@ int ff_id3v2_match(const uint8_t *buf, const char *magic);
 int ff_id3v2_tag_len(const uint8_t *buf);
 
 /**
- * Read an ID3v2 tag, including supported extra metadata
+ * Read an ID3v2 tag into specified dictionary and retrieve supported extra metadata.
+ *
+ * Chapters are not currently read by this variant.
+ *
+ * @param metadata Parsed metadata is stored here
+ * @param extra_meta If not NULL, extra metadata is parsed into a list of
+ * ID3v2ExtraMeta structs and *extra_meta points to the head of the list
+ */
+void ff_id3v2_read_dict(AVIOContext *pb, AVDictionary **metadata, const char *magic, ID3v2ExtraMeta **extra_meta);
+
+/**
+ * Read an ID3v2 tag, including supported extra metadata and chapters.
+ *
+ * Data is read from and stored to AVFormatContext.
+ *
  * @param extra_meta If not NULL, extra metadata is parsed into a list of
  * ID3v2ExtraMeta structs and *extra_meta points to the head of the list
  */