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

[NicTanghe]

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 xmlns="http://www.w3.org/2000/svg" width="128" height="128" viewBox="0 0 128 128">
  <defs>
    <mask id="circle-mask" mask-type="alpha"
      maskUnits="userSpaceOnUse" x="0" y="0" width="128" height="128">
      <circle cx="64" cy="64" r="48" fill="white" />
    </mask>
  </defs>

  <rect width="128" height="128" fill="#2563eb"
    mask="url(#circle-mask)" />
</svg>