C API: add optional Skrifa draw and paint features

[behdad]

Add optional draw and paint features backed by published Skrifa, using the existing face/font handles and C library. paint enables draw; both remain off by default.

Drawing supports unhinted glyf, CFF/CFF2 and VARC outlines at the font's variation coordinates and X/Y scales. Painting supports COLRv0/v1 palettes, gradients, transforms, clips, composites, cached glyph callbacks, SVG, and CBDT/sbix images. Custom font callbacks and parent-font inheritance work with both features. Failed paint traversal unwinds outstanding scopes, and clients can retain image blobs beyond a callback.

Native hr-draw.h / hr-paint.h headers and HarfBuzz compatibility headers expose the APIs. Callback faces use a cached rendering SFNT without requiring table enumeration. The painting adapter follows HarfBuzz's Rust font adapter and preserves its license. Packed color definitions stay in the core API, and the core header is unchanged.

Validation after merging current main and the released C API stack:

• Full Rust suite: 6,074 shaping tests and 160 C API tests pass with all features. Default and draw-only C API suites also pass.
• 2,862 outline/state comparisons against HarfBuzz, including CFF/CFF2, VARC, variable coordinates, zero/negative scales, and cubic fallback; 48 semantic paint comparisons pass.
• All 63 released subset comparisons pass; the combined stack also passes all 79 comparisons with the plan extension enabled.
• GCC/Clang C99 and C++11 rendering consumers pass with shared and static linking. C++11/C++17 ownership/include-order consumers and the same C99 rendering consumer against HarfBuzz pass.
• Rust 1.85, formatting, Clippy, generated headers, and default/draw-only/subset+paint exports pass. Linux/macOS CI checks the rendering combinations and consumers.

Documented limits remain: hinting, bitmap masks, shape convenience functions, configurable budgets, and gradient preprocessing/tiling helpers are not mapped. Skrifa's traversal limits apply. VARC transform precision and bitmap extent rounding may differ slightly from native HarfBuzz; rendering tables are copied once per face.

Stacked on #546 using published dependencies. Merge order: #544 → #545 → #546 → #519 → #547. Only the final draft #547 waits for a Skera release.