The seekable format splits compressed data into a series of independent
frames, each of which can be decompressed individually. This allows to
distribute images in smaller chunks and allows image downloads to be
paused and resumed later from the same point.
Seekable archives as a whole can be decompressed with any regular zstd
decompressor. However, partial decompression requires to know the
starting position of the desired frame, which can be extracted from a
skippable frame (aka seektable) that is appended to the compressed data.
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