postscript: fix initialization of POSTSCRIPT-STREAM
postscript: fix initialization of POSTSCRIPT-STREAM

Before this change, PERMANENT-MEDIUM-SHEET-OUTPUT-MIXIN preceded
STANDARD-EXTENDED-OUTPUT-STREAM in the superclass list. This caused
the following method ordering:

  1. initialize-instance :after standard-page-layout
  2. initialize-instance :after permanent-medium-sheet-output-mixin

Since the first methodism expects the stream's medium to be
initialized, this resulted in an error.

slyrus/mcclim,slyrus/mcclim