beginTransparencyGroup
open fun beginTransparencyGroup(bbox: Rectangle, ctm: Matrix, isolated: Boolean = false, knockout: Boolean = false, alpha: Double = 1.0, blendMode: BlendMode = BlendMode.Normal)
Open a transparency group (ISO 32000-1 §11.4). Subsequent paints accumulate into an offscreen layer; endTransparencyGroup composites the layer back onto the parent with blendMode + alpha.
isolated = the group composites onto a transparent backdrop (true) vs. the parent's current contents (false). knockout is a per-group compositing flavour that's rare outside design PDFs; backends without knockout support fall back to non-knockout.
Default implementation: a no-op pair so backends that don't model groups still render their content (just without isolation). That's incorrect for fancy compositing but produces something visible.