fix: delegate scene.Renderer rasterization to SoftwareRenderer

[kolkov]

Summary

• Delegate scene.Renderer rasterization to gg.SoftwareRenderer — replace broken internal rasterizer with delegation to the existing CPU rendering infrastructure
• Fixes 6 bugs: no AA, stroke ignoring curves (CubicTo/QuadTo), background destruction, no alpha compositing, stubbed layers/clips, only solid brushes
• Zero public API changes — orchestration (TileGrid, WorkerPool, DirtyRegion, LayerCache) preserved

Fixes #124 (reported in #116)

Details

The scene.Renderer had its own simplified rasterizer written before gg.SoftwareRenderer and the GPU pipeline existed. Following the universal pattern confirmed by Qt Quick, Skia, Vello, and Flutter/Impeller — scene graph orchestrates, immediate-mode backend rasterizes — the pixel rendering is now delegated to SoftwareRenderer.

Changed:

• fillPathOnTile / strokePathOnTile → delegate to sr.Fill() / sr.Stroke()
• compositeTile → premultiplied source-over alpha blending (was raw copy())
• sync.Pool for per-tile SoftwareRenderer and Pixmap reuse
• Path conversion: scene.Path (float32) → gg.Path (float64) with tile offset
• Removed dead code: fillPathOnTile, strokePathOnTile, drawLineOnTile, blendPixel

Docs:

• CHANGELOG.md, ROADMAP.md updated for v0.29.4
• docs/ARCHITECTURE.md — new Scene Renderer section

Test plan

• 11 new pixel-level tests (circle fill/stroke, background preservation, alpha compositing, rect fill, transforms, path/paint conversion)
• All 28 existing+new scene tests pass
• go build ./... clean (GOWORK=off)
• go test ./... — 38 packages OK
• golangci-lint run — 0 issues
• go fmt — clean

[codecov]

Codecov Report

❌ Patch coverage is 92.24138% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
scene/renderer.go	92.24%	6 Missing and 3 partials ⚠️

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