Function chrome_damage 

pub fn chrome_damage(
    shown: &Shown,
    cache: &FrameCache,
    size: (u32, u32),
) -> Option<Vec<FineDamageRect>>

Where the chrome texture, holding shown’s chrome, must be rasterised again to hold cache’s, on an output of size: the rects FrameCache::chrome_damage names, when the texture holds the drawing they were measured from. None when that is not known: the whole layer.