Finish VARC through Face: nested reset, self-reference, a shared work budget, short delta sets #296

Follow-up to #291 and #293. These need src/face/outlines.rs, which #289 (shaping cache) is changing, so they wait for #289 to land.

• A VARC component with RESET_UNSPECIFIED_AXES one composite down inherits its parent's coords when drawn through Face::glyph_outline_at_coords, instead of starting from the font's. Pass the top-level coords through glyph_outline_at_coords_inner and call Varc::composite_with_font_coords (added in Fix the known JPEG, COLRv1, VARC and morx bugs #293).
• A VARC component that names its own glyph recurses until the depth cap and fails; HarfBuzz draws the plain outline. Skip VARC when a component's glyph equals its parent's.
• The VARC work budget added in Fix the known JPEG, COLRv1, VARC and morx bugs #293 is per composite_with_font_coords call, and one Face glyph draw can make up to 2,049 such calls. Share one budget across the whole Face call.
• A VARC delta set that ends before it fills its values adds nothing, where HarfBuzz adds what it finds (up to 2.19 units off on a crafted font). Update the sigilbuzz-subset test that encodes the current behavior.