cache prepared SVG shapes for faster renders

[treeform]

Summary

• Fluffy profiling of Ghostscript Tiger showed most newImage(svg) time in fillShapes scanlines, with tessellation and segment partitioning rebuilt on every path every render.
• Added PreparedShapes so solid fills can reuse cached scanline partitions, plus polygonsForFill / polygonsForStroke helpers.
• SVG now lazily prepares solid fills and strokes once, then repeated newImage(svg) calls skip path tessellation, paint allocation, and color reparsing.

Benchmark

nim r -d:danger tests/bench_svg.nim (Ghostscript Tiger, parse once / render many):

• Before (master): ~7.3 ms min
• After (pixie-grok): ~5.8 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)

Made with Cursor

[treeform]

Closing: the SVG-level PreparedShapes cache only helps repeated newImage(svg) on the same parsed Svg. The bench times that path; one-shot tiger renders do not benefit. Will pursue real rasterizer hot-path work instead.