A filter or importer that changes a gradient paint unlinks each stop from
its swatch and resets its tint, but left the stop's process plates. The
PDF and EPS writers read plates only with a spot swatch, so the stop kept
a plates array with no ink, saved with the document and compared in later
edits. Unlinking now clears the plates too, in fills and strokes; a paint
returned unchanged keeps its link, tint and plates.
The plate clamping test now checks the decoded output, before admission.