TLShapeUtilCanvasSvgDef

A <defs> entry a shape needs present in the canvas SVG — a gradient, a filter, a marker, a pattern.

Definitions are collected across every registered util, de-duplicated by key, and rendered once for the whole canvas: ten thousand arrows share one arrowhead marker rather than carrying one each. key is therefore the identity of the definition, not of the shape, and two utils that return the same key must mean the same definition.