applySoftMask
open override fun applySoftMask(kind: SoftMask.Kind, maskBBox: Rectangle, maskCtm: Matrix, render: () -> Unit, renderMask: (KiteCanvas) -> Unit)
Soft-mask compositing (ISO 32000-1 §11.6.5). We open a saveLayer for the content, render it, then over-paint the mask group with ComposeBlendMode.DstIn so the mask's alpha clips the content.
Honest scope: this implements the Alpha SMask kind correctly. The Luminosity kind would require a colour-to-alpha filter (a Skia ColorFilter we don't currently set up); we render it as-if-Alpha, which produces visually plausible results for mask groups whose content is already monochrome-with-alpha (the common case).