Graphite reads clip_path but ignores usvg::Group::mask()

## Description

Graphite’s SVG importer reads resolved clip paths from `usvg::Group::clip_path()`, but
it never reads resolved masks from `usvg::Group::mask()`.

When `usvg` parses an element with `mask="url(#mask-id)"`, it places the resolved mask
on a generated `usvg::Group`. Graphite recursively imports that group’s visible
children while silently discarding its mask. Consequently, the unmasked source artwork
is imported.

## Reproduction

Import `HameronsDocs/usvg-group-mask-import-debug.svg`.

The fixture deliberately contains only:

- One solid blue rectangle.
- One circular SVG mask.
- `mask-type="alpha"` to exclude luminance-mask behavior from this issue.
- No gradients, transforms, strokes, clip paths, filters, or fill-rule dependencies.

```svg

```

Start by importing HameronsDocs/usvg-group-mask-import-debug.svg and trace the SVG importer’s handling of usvg::Group::clip_path() alongside usvg::Group::mask(). Done means the circular alpha mask is preserved so the blue rectangle is clipped to the circle rather than importing the unmasked artwork.

Vom Indexierungsmodell aus dem Issue-Text verfasst.