beginTransparencyGroup
open override fun beginTransparencyGroup(bbox: KiteRectangle, ctm: KiteMatrix, isolated: Boolean, knockout: Boolean, alpha: Double, blendMode: KiteBlendMode)
Transparency groups: open a Compose saveLayer on the active Canvas with a Paint that carries the requested alpha + blend mode. Subsequent draws accumulate into the offscreen layer; matching endTransparencyGroup calls restore, which composites the layer onto the parent.