A .ai swatch library without layers comes in as its PDF part with the
swatches of its editing data's palette. A pattern swatch was kept only
when the PDF part defined its pattern, and a pattern library's PDF part
usually defines none, so its pattern swatches were dropped.
ai::palette now gives pattern swatches for the patterns the editing
data defines too, and returns the definitions the PDF part lacks that a
palette swatch paints with, with the images their art shows. The
document gets them with new object ids.