Support SVG filters via targeted fallback rendering

[rowland]

Problem

Some assets warn with svg: <filter>: unsupported element skipped. SVG filters cover a wide range of effects, and skipping them can materially change the appearance.

What implementation would require

• Identify which filter primitives are actually present in the asset corpus instead of aiming for the whole SVG filter spec.
• Decide on a pragmatic rendering strategy. Native PDF equivalents exist for very little of this, so localized rasterization is the most likely implementation path.
• Add parser support for <filter> references and enough structure to know when a node needs fallback rendering.
• Build a renderer path that isolates the filtered subtree, rasterizes it at a controlled resolution, and places it back into the PDF.
• Add guardrails around output size and performance.

Workarounds

• Pre-rasterize filtered assets to PNG.
• Export SVG variants with filter effects expanded or removed.
• Keep pure-vector SVG only for assets that do not depend on drop shadows, blurs, or similar effects.