wiki: Add expanding circle example to window-open

wiki/examples/open_custom_shader.frag

@@ -105,9 +105,24 @@ vec4 default_open(vec3 coords_geo, vec3 size_geo) {
     return color;
 }
 
+// Example: show the window as an expanding circle.
+// Recommended setting: duration-ms 250
+vec4 expanding_circle(vec3 coords_geo, vec3 size_geo) {
+    vec3 coords_tex = niri_geo_to_tex * coords_geo;
+    vec4 color = texture2D(niri_tex, coords_tex.st);
+
+    vec2 coords = (coords_geo.xy - vec2(0.5, 0.5)) * size_geo.xy * 2.0;
+    coords = coords / length(size_geo.xy);
+    float p = niri_clamped_progress;
+    if (p * p <= dot(coords, coords))
+        color = vec4(0.0);
+
+    return color;
+}
+
 // This is the function that you must define.
 vec4 open_color(vec3 coords_geo, vec3 size_geo) {
     // You can pick one of the example functions or write your own.
-    return solid_gradient(coords_geo, size_geo);
+    return expanding_circle(coords_geo, size_geo);
 }