made luminance work as a mask

[NicTanghe]

Summary

Adds typed support for distinguishing alpha and luminance clipping masks throughout
Graphite’s document and rendering pipeline.

Previously, all clipping masks were treated as alpha masks. Consequently, opaque black
and white SVG mask artwork produced identical results, even though luminance masks
should use color luminance multiplied by alpha.

When merging take special attention to how it interacts with goupmasks becouse it should transfer extra fields ecs.

[gemini-code-assist]

Code Review

This pull request introduces support for luminance masks alongside the existing alpha masks. It defines a new MaskMode enum (with Alpha and Luminance variants), adds a mask_mode node in the blending category to configure this property on graphics, and updates both the SVG and Vello renderers to handle luminance masking. There are no review comments, so I have no feedback to provide.

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[cubic-dev-ai]

All reported issues were addressed across 7 files

Confidence score: 5/5

• Safe to merge after the addressed issues were fixed.

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[NicTanghe]

Was only able to get the luminance masks working after merging the 2 branches.