teach old-style method about the new form of screen numbering fdo#43458

We number now with a flat set of indicees across Xinerama
and more standard multi-screen setups.

vcl/unx/gtk/window/gtkframe.cxx

@@ -4162,12 +4162,8 @@ gboolean GtkSalFrame::IMHandler::signalIMDeleteSurrounding( GtkIMContext*, gint
 
 Size GtkSalDisplay::GetScreenSize( int nScreen )
 {
-    GdkScreen *pScreen = gdk_display_get_screen (m_pGdkDisplay, nScreen);
-    if (!pScreen)
-        return Size();
-    else
-        return Size( gdk_screen_get_width (pScreen),
-                     gdk_screen_get_height (pScreen) );
+    Rectangle aRect = m_pSys->GetDisplayScreenPosSizePixel( nScreen );
+    return Size( aRect.GetWidth(), aRect.GetHeight() );
 }
 
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