Function extract_opacity_groups 

pub fn extract_opacity_groups(main: &mut World, render: &mut World)

Extracts one ExtractedOpacityGroup per element that fades as a group (see opacity_model). Each carries the element’s opacity and the (start_order, end_order) bracket of the paints it fades, which crate::node_ir::transform_extracted_to_nodes folds into a crate::node_ir::Node::Opacity.