Support SVG masks

[rowland]

Problem

Some assets warn with svg: <mask>: unsupported element skipped. Skipping masks can remove large portions of the intended artwork.

What implementation would require

• Parse <mask> definitions from <defs> and resolve mask="url(#...)" references on painted nodes.
• Support the mask coordinate systems and units needed by the current asset set.
• Decide whether masks will be rendered via native PDF soft masks or by localized rasterization fallback.
• Clarify how mask content interacts with opacity and clipping support.
• Add sample assets and visual tests for common alpha-mask cases.

Workarounds

• Flatten masked artwork to PNG before embedding.
• Replace masked compositions with equivalent clipped/vector artwork when possible.
• Export alternate SVGs that avoid masks for PDF use.