[Editor] Add support for printing/saving newly added Stamp annotations

In order to minimize the size the of a saved pdf, we generate only one
image and use a reference in each annotation using it.
When printing, it's slightly different since we have to render each page
independantly but we use the same image within a page.

src/shared/util.js

@@ -70,6 +70,7 @@ const AnnotationEditorType = {
   DISABLE: -1,
   NONE: 0,
   FREETEXT: 3,
+  STAMP: 13,
   INK: 15,
 };