- Core compositor gains an optional screen-blend tint per CompositorLayer
([0,0,0,0] default = no-op, so existing compositing is unaffected);
CompositorLayer::with_tint sets it — giving the Vello/vector path a tint hook.
- For the active VECTOR layer with onion on, build ghost scenes at each neighbouring
keyframe's time via render_layer_isolated (reusing the prepare's image cache), then
render each scene → sRGB → linear → composite with warm/cool tint + opacity falloff,
behind the current frame. Off during playback; active layer only.
Completes onion skinning for all layer types (raster + vector).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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