Emit PageOrientation DSC comment into PostScript print output

The PageOrientation DSC comment tells PostScript viewers such as gv or
evince the orientation to use when displaying the page. Part of bug 596423

gtk/gtkprintoperation-unix.c

@@ -91,9 +91,26 @@ unix_start_page (GtkPrintOperation *op,
       (op->priv->page_position % op->priv->manual_number_up == 0))
     {
       if (type == CAIRO_SURFACE_TYPE_PS)
-        cairo_ps_surface_set_size (op_unix->surface, w, h);
+        {
+          cairo_ps_surface_set_size (op_unix->surface, w, h);
+          cairo_ps_surface_dsc_begin_page_setup (op_unix->surface);
+          switch (gtk_page_setup_get_orientation (page_setup))
+            {
+              case GTK_PAGE_ORIENTATION_PORTRAIT:
+              case GTK_PAGE_ORIENTATION_REVERSE_PORTRAIT:
+                cairo_ps_surface_dsc_comment (op_unix->surface, "%%PageOrientation: Portrait");
+                break;
+
+              case GTK_PAGE_ORIENTATION_LANDSCAPE:
+              case GTK_PAGE_ORIENTATION_REVERSE_LANDSCAPE:
+                cairo_ps_surface_dsc_comment (op_unix->surface, "%%PageOrientation: Landscape");
+                break;
+            }
+         }
       else if (type == CAIRO_SURFACE_TYPE_PDF)
-        cairo_pdf_surface_set_size (op_unix->surface, w, h);
+        {
+          cairo_pdf_surface_set_size (op_unix->surface, w, h);
+        }
     }
 }