beginTransparencyGroup
open override fun beginTransparencyGroup(bbox: KiteRectangle, ctm: KiteMatrix, isolated: Boolean, knockout: Boolean, alpha: Double, blendMode: KiteBlendMode)
ISO 32000-1, 11.4.5: a group's constant alpha and blend mode apply once, when the group composites onto its backdrop. So a group that has them paints into a layer first (#77). The layer is isolated, and knockout is not honoured (#125).