fbpdf: O command sets page number and shows current page

For instance, the command '3O' in page 5 of a file:
1. Updates current page number so that the command 'nG'
   shows page n + 2, where n is the prefix (just like the
   command '3o').
2. Invokes command G with current page as its prefix, i.e.,
   '5G', which shows page 7.