Grok: speed up path fill scratch and solid SVG paints

[treeform]

Summary

• Closed the earlier SVG geometry-cache approach: that only helped repeated newImage(svg) on the same parsed Svg, not one-shot tiger renders.
• This PR speeds up the real rasterizer path used on every fill:
  • Precompute invM for partition edge solves (multiply instead of divide).
  • Reuse per-thread fillScratch buffers across the many paths inside one SVG render (segments, partitions, hits, coverages).
  • Clip partition edges with solveX instead of line intersects.
  • Add solid-color fillPath / strokePath overloads so SVG solid fills skip Paint allocation.

Why this helps one-shot renders

A single Ghostscript Tiger render fills ~240 paths. Scratch reuse amortizes allocations within that one render, not across bench iterations. invM and Paint avoidance also pay off on the first (and only) draw.

Benchmark

nim r -d:danger tests/bench_svg.nim:

• master: ~6.80 ms min
• this branch: ~6.61 ms min
• target to beat: 6.834 ms

Test plan

• nim r -d:danger tests/bench_svg.nim
• nim r -d:danger tests/test_svg.nim (xray masters match)

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