fillShading
open fun fillShading(shading: PdfShading, ctm: Matrix, clipPath: PdfPath?, alpha: Double = 1.0, blendMode: BlendMode = BlendMode.Normal)
Fill clipPath (under ctm) with the gradient defined by shading. If clipPath is null the shading covers the current clip region (or the page, when no clip is active) — the spec's sh operator.
Default impl falls back to a flat-colour fill using the midpoint sample; backends that can render real gradients override.