fix(svm): draw through a graphics state stack, and stop the fill killing the stroke

[andiwand]

🤖 Generated with Claude Code

Stage 2 of #772, stacked on #779 — review that one first; this branch's
base is feat/svm-svg-writer. It fixes the defects in what we already draw,
before any new action is added.

The graphics state stack (#772 defect 2)

PUSH and POP fell through to default: like any unimplemented action, so
there was no state stack: a colour, font or map mode set inside a push leaked
out of it and contaminated the rest of the drawing.

This is the most common thing we get wrong. Harvesting every embedded metafile
from the odt/ods fixtures — 1125 of them — gives:

action	occurrences	files (of 1125)
PUSH / POP	22317 each	1124
TEXTALIGN	21534	1117
STRETCHTEXT	20335	1097
ISECTRECTCLIPREGION	1124	1123

The flags matter as much as the stack: 20919 of those 22317 pushes save five of
the fourteen state groups (LINECOLOR|FILLCOLOR|FONT|TEXTCOLOR|MAPMODE), only
1140 save everything. Restoring the whole state on a pop would therefore be
wrong more often than restoring nothing, so PushFlags is modelled and a pop
puts back exactly what its push asked for.

Four more, all in what we already drew

• The fill killed the stroke. A rect or polygon wrote the line style and
  then the fill style, and the fill style ended in stroke:none — so a filled
  shape never had an outline. In vcl the pen outlines what the brush fills.
• A poly-polygon is one shape (Improve SVM to SVG conversion #772 defect 3). It was one <polygon> per
  sub-polygon, so donuts, rings and letter counters came out solid. Now one
  <path> with fill-rule:evenodd — which is what vcl fills with, and what
  LibreOffice's own svg export puts on its root element.
• LineInfo was read and dropped (Improve SVM to SVG conversion #772 defect 8). Width, dash pattern and
  join now reach the stroke; a default LineInfo still gets the hairline.
• The font size did not scale (Improve SVM to SVG conversion #772 defect 5). Coordinates went through the
  map mode and font-size did not, so text kept its size while the drawing
  shrank around it.

svgwriter.cxx's GetPathString is the model for the path data — which also
puts the bézier stage within reach, since its C segments are the same
function.

What is not here

The map mode's unit (#772 defect 6) is deferred to its own stage, and
svm/PLAN.md says why: MetaMapModeAction::Execute calls SetMapMode, which
replaces the map mode except when the new unit is MapRelative, where scales
multiply and origins add. We do neither. It is 4 actions in 1125 files and
wants vcl/source/outdev/map.cxx read properly rather than guessed at.

Verification

7 new tests (15 in the svm suite now), each built from inline bytes: a shape
draws both colours, an unset colour draws nothing, a pop restores what its push
saved, a pop keeps what its push did not save, a poly-polygon is one path
with the fill rule, a polyline takes its LineInfo, and the font size scales.

Rendering: A/B'd the four svm fixtures and the changed pages of the corpus in
headless Chrome. The charts and the 1032 formulas of
odt/Vektoranalysis Zusammenfassung.odt come out equivalent; test.svm's data
series is now drawn at the width the file asks for instead of as a hairline,
matching what LibreOffice renders.

This changes the emitted html for 23 files, so the reference output has to be
regenerated and the pin in test/data.cmake advanced before CI's compare step
can pass. All 23 are documents that carry an svm; the public reference set is
untouched.