Paints a shading over the whole current clip (the sh operator).
void DrawShading(in DisplayShading shading)
The shading, its geometry mapped to default user space by Matrix (the CTM at sh), with
PaintsBackground false.
ISO 32000-2 §8.7.4.2, Table 76: the shading fills the current clipping path, further clipped by its BBox when it has one (§8.7.4.3); points the shading does not define (beyond an end that is not extended, §8.7.4.5.3 and §8.7.4.5.4) are left unpainted; the Background is ignored. The current colour plays no part.