flush surface after 'draw'

on f21 otherwise we see nothing under gtk3, f20 doesn't need it FWIW.

Change-Id: I409c828644bff793f4a82ecc7d4cad4da1eeb8da

vcl/unx/gtk/window/gtksalframe.cxx

@@ -3495,6 +3495,8 @@ gboolean GtkSalFrame::signalDraw( GtkWidget*, cairo_t *cr, gpointer frame )
 
     pThis->m_nDuringRender--;
 
+    cairo_surface_flush(cairo_get_target(cr));
+
     return FALSE;
 }
 #endif // GTK_CHECK_VERSION(3,0,0)