applySoftMask
open override fun applySoftMask(kind: SoftMask.Kind, maskBBox: KiteRectangle, maskCtm: KiteMatrix, render: () -> Unit, renderMask: (KiteCanvas) -> Unit)
open override fun applySoftMask(kind: SoftMask.Kind, maskBBox: KiteRectangle, maskCtm: KiteMatrix, transfer: KiteMaskTransfer?, render: () -> Unit, renderMask: (KiteCanvas) -> Unit)
ISO 32000-1, 11.6.5.2: the content and the mask group each paint into a layer of their own. The mask layer's alpha, or for a luminosity mask its luminosity over a black backdrop, gates the content layer, which then composites onto the page. The mask group's colours never reach the page (#161). The transfer table maps each mask value.