SVG picture fallback: restate the root's size with ooxml-common's svgimage

[uvrt]

What

edit/svgpicture.rasterise used its own workaround for resvg-py's "SVG has an invalid size". It parsed the SVG with lxml, set width/height to native_size (or to the placed size when the SVG stated none), and re-serialised the whole tree. It now calls the shared ooxml_common.svgimage.with_pixel_size (ooxml-common 0.12.0, uvrt/ooxml-common#23). pptx2svg now uses the same function for the SVG pictures inside a page (uvrt/pptx2svg#25).

Behaviour

• px, pt, pc, in, cm, mm: the same pixels as before.
• em: 12 px, or the root's font-size. This is PowerPoint's value (CSS uses 16, and the old code skipped em entirely).
• % or no size: the viewBox's size, or 500 px when there is no viewBox, where the placed size was used before. All measured on PowerPoint 16's PDF export: 30 of 30 probes match (see pptx2svg's tools/make_svg_size_probe.py). The fallback PNG now matches what PowerPoint draws from the vector.
• Rewrite scope: only those attributes in the root start tag change. The SVG is no longer re-serialised through lxml.
• Encoding: a Latin-1 or UTF-16 SVG is re-encoded instead of refused.
• Unchanged: native_size, which sizes the frame when the picture is inserted.

Tests

• New: the fallback draws pt, cm/in and pc/mm-sized SVGs (both halves of a split-colour viewBox are in place).
• New: a no-viewBox 6em SVG is drawn with PowerPoint's 12 px em. This one fails before the change.
• Full suite: 2358 passed, 29 skipped.

Merge order

1. SVG pictures: natural size as PowerPoint reads it, restated in pixels (0.12.0) ooxml-common#23 first.
2. Then point .github/workflows/ci.yml back at @main (the # TEMPORARY (gaps) line) and merge this. It does not depend on Draw an SVG picture in PNG output whatever unit its root's size is in pptx2svg#25.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WCxhY1fvWQPiWCwz7SYUVh