Minimize GtkSalData scope in timeout callback

After commit 2e8a95d1 this doesn't
anymore generate false positives in the constparams clang plugin:

vcl/unx/gtk/gtkdata.cxx:817:39: error: ...
        ... this parameter can be const [loplugin:constparams]
    static gboolean call_userEventFn( void *data )
                                      ^~~~~~~~~~
1 error generated.

/vcl/unx/gtk3/gtk3gtkdata.cxx:782:39: error: ...
        ... this parameter can be const [loplugin:constparams]
    static gboolean call_userEventFn( void *data )
                                      ^~~~~~~~~~
1 error generated.

Change-Id: Id8d42a0c14bfcc887ec1ecbf70f4c41c26e26efb
Reviewed-on: https://gerrit.libreoffice.org/42912Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>

vcl/unx/gtk/gtkdata.cxx

@@ -816,12 +816,11 @@ void GtkSalTimer::Stop()
 extern "C" {
     static gboolean call_userEventFn( void *data )
     {
-        GtkSalData *pThis = static_cast<GtkSalData *>(data);
         SolarMutexGuard aGuard;
         const SalGenericDisplay *pDisplay = GetGenericUnixSalData()->GetDisplay();
         if ( pDisplay )
         {
-            GtkSalDisplay *pThisDisplay = pThis->GetGtkDisplay();
+            GtkSalDisplay *pThisDisplay = static_cast<GtkSalData *>(data)->GetGtkDisplay();
             assert(static_cast<const SalGenericDisplay *>(pThisDisplay) == pDisplay);
             pThisDisplay->DispatchInternalEvent();
         }

vcl/unx/gtk3/gtk3gtkdata.cxx

@@ -781,12 +781,11 @@ void GtkSalTimer::Stop()
 extern "C" {
     static gboolean call_userEventFn( void *data )
     {
-        GtkSalData *pThis = static_cast<GtkSalData *>(data);
         SolarMutexGuard aGuard;
         const SalGenericDisplay *pDisplay = GetGenericUnixSalData()->GetDisplay();
         if ( pDisplay )
         {
-            GtkSalDisplay *pThisDisplay = pThis->GetGtkDisplay();
+            GtkSalDisplay *pThisDisplay = static_cast<GtkSalData *>(data)->GetGtkDisplay();
             assert(static_cast<const SalGenericDisplay *>(pThisDisplay) == pDisplay);
             pThisDisplay->DispatchInternalEvent();
         }