When a MOBI or AZW3 file names no cover (no EXTH 201 or 202), the cover is
the record at the first image index if it is a JPEG, PNG or GIF, and
nothing otherwise, as calibre does (metadata/mobi.py:524-532). The walk to
the next image is gone: a later record may be an inline illustration, and
kindlegen puts a font or RESC record first.
EXTH 121 is no longer read, which also stops a stale KF8 boundary below
first_image from hiding the cover.
funkpower
a105b1c9aa