cre 2-pages view: allow extending selection across pages

Similar to what's been added for 1 page view, but just turn
one page instead of switching to scroll mode when reaching
top left or bottom right corners.
Also make the selection start xpointer more accurate by
getting them in onHold(), instead of possibly too late in
onHoldPan() where we have already moved.

frontend/document/credocument.lua

@@ -280,6 +280,11 @@ function CreDocument:getWordFromPosition(pos)
             w = 20, h = 20,
         }
     end
+    if text_range then
+        -- add xpointers if found, might be useful for across pages highlighting
+        wordbox.pos0 = text_range.pos0
+        wordbox.pos1 = text_range.pos1
+    end
     return wordbox
 end