Function scrim_maps 

pub fn scrim_maps(value: u8, strength: f32, theme: &Theme) -> u8

What fill_scrim does to one 8-bit grey behind it, as a pure function so the property the two fills are for can be asserted without a GPU. Must mirror the shader: soft light against black is Cb² (W3C compositing) composited lerp(Cb, Cb², αs), then an ordinary source-over of the theme’s scrim hue for the level stage.