avformat/id3v2: Avoid allocations for ID3v2ExtraMeta

avformat/id3v2: Avoid allocations for ID3v2ExtraMeta

Up until now, the ID3v2ExtraMeta structure (which is used when parsing
ID3v2 tags containing attached pictures, chapters etc.) contained a
pointer to separately allocated data that depended on the type of the
tag. Yet the difference of the sizes of the largest and the smallest of
these structures is fairly small, so that it is better to simply include
a union of all the possible types of tag-dependent structures in
ID3v2ExtraMeta. This commit implements this.

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