struct TransportMark

Media transport symbols — play, pause, previous, next — drawn as filled shapes, for the same reason Mark is not a character: "▶" and "⏮" are whatever the fallback font made of them, sat on its baseline, at a size and weight that never match each other, and "❚❚" is two letters.

Geometry rather than a picture: a few triangles and rectangles cost what a glyph does, need no texture, stay sharp at any scale, and take the theme's colour as it changes. No input of its own, as Mark.