Gpt: Speed up path rasterization

[treeform]

Speeds up the SVG path rasterizer by:

• caching reciprocal slopes for scanline coverage
• computing each segment partition range once
• reusing partition counts as insertion indexes
• passing the active partition directly into coverage calculation

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

• before: 6.930 ms minimum
• after: 6.688 ms minimum
• repeated final minima: 6.688, 6.749, 6.723 ms

Verified with:

• nim check tests/test_svg.nim
• nim r -d:danger tests/test_svg.nim
• nim r tests/tests.nim
• git diff --check