struct ShapedGlyph

A shaped glyph, plus which face has to draw it.

canvas.PositionedGlyph carries no face, because it predates fallback: one run meant one face and the emitter stamped the run's id onto every glyph. With substitution a single run can mix faces, and glyph ids are face-relative — shipping the id alone would draw the wrong glyph. Field names deliberately mirror PositionedGlyph so everything doing caret and advance arithmetic reads unchanged.