Give the image overlay an optional shadow or glow

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>

src/effects/overlay.hpp

@@ -11,7 +11,8 @@ namespace effector {
 
 // Lays one picture from disk over its input: a logo, a texture, a frame. The
 // overlay is fitted to the canvas in the chosen way, placed from the top left
-// or from the centre, nudged by an offset and mixed in by its opacity. Like the
+// or from the centre, nudged by an offset and mixed in by its opacity, with a
+// soft shadow or glow cut from its own outline beneath it if asked. Like the
 // blur it needs no source images of its own, so it can lead a chain; given
 // sources it fills the canvas from a randomly chosen one first.
 class OverlayEffect : public Effect {
@@ -20,7 +21,8 @@ public:
     std::string title() const override { return "Image overlay"; }
     std::string description() const override {
         return "Lays a single image from disk over its input, fitted to the canvas, centred or "
-               "not, nudged by an offset and mixed in by its opacity.";
+               "not, nudged by an offset and mixed in by its opacity, optionally with a shadow or "
+               "a glow around it.";
     }
     const std::vector<ParamSpec>& params() const override;
     bool needsSources() const override { return false; }