feature: adapt to image property of custom style for custom style ico…

…n in legend (#1961)

src/controls/legend/imagesource.js

@@ -1,11 +1,9 @@
 const getSource = (style) => {
-  if ('image' in style) {
-    if (style.image.src) {
-      return style.image.src;
-    }
-    throw new Error('image style must have src');
-  }
-  return null;
+  const image = style.image || style.custom.image;
+  if (!image) return null;
+  const src = image.src || null;
+  if (!src) throw new Error('image style must have src');
+  else return src;
 };
 
 // return the image source from an origo style