Keep the number of lines to flip as a negative num

Keep track of the number of lines to flip on the reverse page using a negative
number.  This uses the ill-advised principle of overloading the meaning of a
variable.  Still, it kind of makes sense because these lines are upside down,
so the number should be "upside down" too.  Besides, leaving it as a positive
number trips up the code which does page padding, which would require even
more special case code.

backend/avision.c

@@ -7486,7 +7486,7 @@ reader_process (void *data)
       }
       /* we can set anything here without fear because the process will terminate soon and take our changes with it */
       s->page += 1;
-      s->params.lines = line;
+      s->params.lines = -line;
       exit_status = reader_process (s);
     }
     /* TODO: