applySoftMask

open override fun applySoftMask(kind: SoftMask.Kind, maskBBox: Rectangle, maskCtm: Matrix, render: () -> Unit, renderMask: (KiteCanvas) -> Unit)

Soft mask (ISO 32000-1 §11.6.5). Content is painted on the live surface, then gated by the mask group:

SoftMask.Kind.Alpha: the mask group's alpha, applied via DST_IN.
SoftMask.Kind.Luminosity: the mask group is rendered offscreen over a black backdrop, its per-pixel luminance is converted to alpha, and that alpha map is DST_IN-composited onto the content.