Fix for 687314 - infinite recursion over circular resource links

Insert a mark into the resource dictionary to do a depth-first search
of the Resource reference graph even when there are circularities.
Fixes bug #687314.

Index: lib/pdf_main.ps
===================================================================
RCS file: /cvs/ghostscript/gs/lib/pdf_main.ps,v
retrieving revision 1.87
diff -C2 -r1.87 pdf_main.ps
*** lib/pdf_main.ps	1 Jun 2004 21:13:04 -0000	1.87
--- lib/pdf_main.ps	23 Jun 2004 23:34:08 -0000
***************
*** 1144,1147 ****
--- 1144,1149 ----
  /resourceusestransparency {	% <dict> resourceusestransparency <bool>
    {	% Use loop to provide an exitable context.
+     dup /.resourceusestransparency_mark known { pop false exit } if
+     dup /.resourceusestransparency_mark true put
      /Resources knownoget not { 0 dict } if
      dup /ExtGState knownoget {