A blurred copy of the overlay's own alpha is drawn in a colour beneath it:
moved by an offset for a shadow, centred for a glow, with a size, a spread
and an opacity. The mask is integer-only so a seed still reproduces it, and
reaches past the canvas so an image lying off it still casts onto it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>