Support context paint in SVG use elements

[moreaki]

Stacked on #2744.

Context

#2744 adds marker-scoped support for the SVG 2 context-fill and context-stroke paint values, fixing the concrete marker case from #2667. The SVG 2 paint definition also says that an element inside a subtree instantiated by a use element has that use element as its context element.

This draft PR adds that second small context-paint case on top of #2744, so maintainers can decide whether they want to include it as a follow-up feature.

Changes

• Reuse the context-paint tracking introduced by Support context paint in SVG markers #2744 when drawing children of a use element.
• Treat the use element as the context element while its referenced subtree is drawn.
• Add a pixel regression test proving that context-fill and context-stroke are resolved independently for multiple use instances with different solid paint values.

Scope and limitations

This intentionally supports the simple solid-color case. It does not try to solve the harder paint-server geometry case for contextual url(#gradient) or url(#pattern) values. For those, SVG 2 requires the paint server coordinate space and bounding box to come from the context element, which needs additional gradient/pattern plumbing beyond this small follow-up.

Until #2744 lands, this PR’s diff includes the marker context-paint groundwork from #2744 plus one additional commit for use elements.

Tests

• venv/bin/python -m pytest tests/draw/svg/test_defs.py::test_use_context_paint tests/draw/svg/test_defs.py::test_use_symbol_color tests/draw/svg/test_markers.py::test_markers_context_paint
• venv/bin/python -m pytest tests/draw/svg
• venv/bin/python -m ruff check weasyprint/svg/__init__.py tests/draw/svg/test_defs.py tests/draw/svg/test_markers.py