Add uhdr example

Author: rolandlo

example/images/uhdr.jpg

@@ -0,0 +1,17 @@
+local vips = require "vips"
+
+local image = vips.Image.new_from_file("images/uhdr.jpg", { access = "sequential" })
+local gainmap = image:get_gainmap()
+print("image:", image)
+
+if gainmap then
+    print("gainmap:", gainmap)
+    --[[ Due to reference sharing in the operation cache copying the image
+         as below is needed when it is not necessarily unique
+      image = image:copy()
+    ]]
+    -- set the "gainmap" metadata for the original image
+    image:set_type(vips.gvalue.image_type, "gainmap", gainmap)
+else
+    print("No gainmap found. You need libvips >= 8.18 with uhdr support")
+end