fix Colour_atlas

Since const ++ class will no longer works

Author: jcupitt

CHANGELOG.md

@@ -10,6 +10,7 @@
 - improve display of computed multipage images
 - batch mode output is lazy
 - ++, join_sep, difference are lazy
+- const ++ Image no longer works :( patch Colour_atlas
 - removed trace system
 
 ## 9.0.13 2025/09/20

TODO

@@ -1,3 +1,5 @@
+- should allow vips_image ++ Image
+
 - remove remove prefs workspace stuff?
 
     things like max heap size should be settable ... maybe pres are useful?

share/nip4/start/Image.def

@@ -2363,7 +2363,10 @@ Test_images_item = class
 				lab_start = colour_transform_to Image_type.LAB start;
 				blocks' = blocks - to_real nstep * to_real ssize +
 					Vector (tl lab_start.value);
-				blocks'' = hd lab_start.value ++ Image blocks';
+                L = image_new size size 1
+					Image_format.FLOAT Image_coding.NOCODING
+					Image_type.B_W lab_start.value?0 0 0;
+				blocks'' = Image L ++ Image blocks';
 				blocks'''
 					= image_set_type Image_type.LAB blocks'', Image grid
 					= 0;