Function threat_overlays 

pub fn threat_overlays(
    threat: &ThreatView,
    grid: &Grid,
    start: Start,
    colors: Colors,
) -> Vec<Overlay>

The overlays drawing threat on grid, all Over the glyphs: every arrow’s dark edge, then the arrows (each from its enemy’s tile at start); then every mark’s dark edge and the marks. Clipped to the tiles of grid. On a zoomed grid every pixel is a square of the zoom’s.