Eeschema printing: ensure the flag "is first page" is correctly set.

It is mainly used to draw page layouts having different objects drawn on
first page and other pages.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/20318

eeschema/printing/sch_printout.cpp

@@ -98,6 +98,7 @@ bool SCH_PRINTOUT::OnPrintPage( int page )
     // Ensure the displayed page number is updated:
     KIGFX::SCH_VIEW* sch_view = m_parent->GetCanvas()->GetView();
     sch_view->GetDrawingSheet()->SetPageNumber( TO_UTF8( screen->GetPageNumber() ) );
+    sch_view->GetDrawingSheet()->SetIsFirstPage( screen->GetVirtualPageNumber() == 1 );
 
     // Print page using the current wxPrinterDC
     PrintPage( screen, GetDC(), true );