advance method
Advance width of glyph, font units: at an instance, with its
HVAR delta, or from its varied phantom points without HVAR.
Implementation
int advance(int glyph) {
if (glyph < 0 || glyph >= glyphCount) return 0;
final base = _hmtxAdvance(glyph);
if (normalizedCoordinates.isEmpty) return base;
return _advances[glyph] ??= _variedAdvance(glyph, base);
}