Tracking issue: Features for Blitz

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### Vello Hybrid parity

- [x] https://github.com/linebender/vello/issues/1373
- [x] External texture support
- [ ] External textures should be usable as a general "paint" source

### Vello CPU (multithreaded) parity

- [x] Should not crash on unsupported features (e.g. image filters should noop rather than panicking if not supported)
- [ ] Actual image filter support

### Rendering Features

- [x] Glyph embolden / stem darkening
- [ ] Make emboldened text less fuzzy (gamma correction?)
- [ ] Inset box shadows (https://github.com/linebender/vello/issues/1374)
- [ ] Elliptical / Superelliptical "blurred rounded rect" corners https://github.com/linebender/vello/issues/1245
- [ ] Image filters
- [x] Blur
- [x] Drop Shadow
- [ ] Pixel-local filters (e.g. color transformations)
- [ ] Other complex filters
- [ ] Backdrop filters
- [ ] Alpha masking

### API Model

- [ ] Global (Scene relative) paint transform (so that a paint can be "fixed" in position and then filled by multiple separate fill commands)
- Use cases: `background-clip: text` and `background-position: fixed`
- [ ] ID-based Resource registration (e.g. images/textures) for CPU and Classic
- [ ] Render Scene to retained texture / atlas slot (= mini render graph?)
- The motivation here is to take advantage of the main renderer to render sub-images that are retained between frames
- Use cases include the CSS `mask` property where a mask (which may be e.g. an SVG) needs to be rasterized and then used within the same frame. But would ideally then be retained for future frames.

### Performance

- [ ] Fast raster image rendering
- [ ] Glyph caching

This is a broad tracking issue with several unchecked rendering, API, and performance goals rather than a named file or entry point. Choose one item first, then read the linked issues for inset box shadows (1374) or blurred rounded-rect corners (1245), where applicable. Done means the selected feature is implemented and its corresponding parity or rendering behavior is verified.

Written by the indexing model from the issue text.