id3demux: fix parsing of unsync'ed ID3 v2.4 tags and frames
Reversing the unsynchronisation seems to work slightly differently for ID3 v2.3 tags and v2.4 tags: v2.3 tags don't have syncsafe frame sizes in the frame header, so the unsynchronisation is applied to the whole frame data including all the frame headers. v2.4 frames have sync-safe sizes, however, so the unsynchronisation only needs to be applied to the actual frame data, and it seems that's what's being done as well. So we need to undo the unsynchronisation on a per-frame basis for v2.4 tags for things to work properly. Fixes extraction of coverart/images from APIC frames in ID3 v2.4 tags (#588148). Add unit test for this as well.
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