A dash pattern with a negative value (for example [-5, 3], from `stroke.set` or a PDF `d`
operator) was handed to the dasher whenever any value was positive. When the values sum to zero
or less the dasher never advances, so rendering the document hung forever (canvas, screenshot,
PNG/JPEG export, `vectorcraft-cli convert`).
PDF (32000-1 §8.4.3.6) and SVG (`stroke-dasharray`) both define such a pattern as invalid and the
stroke as solid. `Dash::is_dashed` now says whether a pattern draws dashes (finite, non-negative,
not all zero), and the renderer, the width-profile outline, the SVG and PDF exporters and the PDF
importer use it, so an invalid pattern is drawn, written and read back as a solid stroke everywhere
(the PDF exporter used to clamp negatives to 0, writing zero-length dashes).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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