Support static VARC variation axes

[behdad]

VARC permits a static font to omit fvar while retaining gvar or CFF2 variation data for component-internal coordinates. Skrifa previously sized the VARC coordinate array only from font.axes(), so a component axis override in such a font failed as malformed.

Derive the internal axis count from gvar or the CFF2 VariationStore and combine it with the public fvar count. The regression fixture has one hidden gvar axis and no fvar; its VARC component sets that axis to 0.5 and the test verifies the resulting 50-unit outline shift.

Validation:

• cargo fmt --all -- --check
• cargo test -p skrifa outline::varc::tests