applySoftMask
open override fun applySoftMask(kind: SoftMask.Kind, maskBBox: Rectangle, maskCtm: Matrix, render: () -> Unit, renderMask: (PdfCanvas) -> Unit)
Soft mask via offscreen compositing. We render the content layer onto an offscreen BufferedImage, render the mask group onto a second one, apply DST_IN, then blit the result onto the underlying Graphics2D.