vcl: isolate new attempt to count monitors as screens to gtk3 fdo#43458

It turns out this new idea of mapping xinerama screens, and xrandr
monitors into a flat integer space is nice, but when we confuse
xrandr monitor 2 (idx 1) with X screen 1 (there is only one screen)
we bit the dust.

Many thanks to Cor Nouws for remote debugging assistance

vcl/inc/unx/gtk/gtkdata.hxx

@@ -154,10 +154,12 @@ public:
     GdkCursor *getCursor( PointerStyle ePointerStyle );
     virtual int CaptureMouse( SalFrame* pFrame );
 
+#if GTK_CHECK_VERSION(3,0,0)
     bool IsXinerama() { return !m_pSys->IsMultiDisplay(); }
     int  GetDefaultScreenNumber() { return m_pSys->GetDefaultDisplayNumber(); }
     int  GetScreenCount() { return m_pSys->GetDisplayScreenCount(); }
     Size GetScreenSize( int screen );
+#endif
     virtual void initScreen( int nScreen ) const;
 
     GdkFilterReturn filterGdkEvent( GdkXEvent* sys_event,

vcl/unx/gtk/window/gtkframe.cxx

@@ -4160,10 +4160,12 @@ gboolean GtkSalFrame::IMHandler::signalIMDeleteSurrounding( GtkIMContext*, gint
     return sal_False;
 }
 
+#if GTK_CHECK_VERSION(3,0,0)
 Size GtkSalDisplay::GetScreenSize( int nScreen )
 {
     Rectangle aRect = m_pSys->GetDisplayScreenPosSizePixel( nScreen );
     return Size( aRect.GetWidth(), aRect.GetHeight() );
 }
+#endif
 
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