Function provide_backdrops 

pub fn provide_backdrops(images: Vec<(usize, ImageData)>)

Hand the next frames the blurred backdrops of their surfaces, by region slot of the outermost recording: a host that cannot blur in its renderer (an app rasterized on the CPU inside a compositor, which holds what lies below) has them made where that is known and drawn in place of the placeholders, so the material’s band and tint apply to them as to a substituted blur. An image is used only while its size is the slot’s placeholder size (backdrop_texels); the rest stay opaque.