Pass ActualText through to BDC properties in content stream

Screen readers read BDC properties from the content stream, not just
the structure tree. When ActualText is set on a structure element,
include it in the BDC marked content properties so screen readers
can use it as replacement text.

Author: craigmcnamara

lib/pdf/core/structure_tree.rb

@@ -149,7 +149,10 @@ def mark_content(tag, struct_elem_ref: nil)
         @parent_tree_map[page_struct_parents][mcid] = elem_ref
 
         # Emit BDC/EMC in content stream
-        renderer.begin_marked_content_with_properties(tag, { MCID: mcid })
+        bdc_props = { MCID: mcid }
+        actual_text = renderer.deref(elem_ref)[:ActualText]
+        bdc_props[:ActualText] = actual_text if actual_text
+        renderer.begin_marked_content_with_properties(tag, bdc_props)
         yield if block_given?
         renderer.end_marked_content
       end