Use the backend SalInstance method IsMainThread for Application::IsMainThread

Change-Id: I3258268b12cb0914a5e8d1c9b57d7891e83080c6
Reviewed-on: https://gerrit.libreoffice.org/62951Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins

vcl/inc/svdata.hxx

@@ -363,7 +363,6 @@ struct ImplSVData
     std::unique_ptr<DockingManager> mpDockingManager;
     std::unique_ptr<BlendFrameCache> mpBlendFrameCache;
 
-    oslThreadIdentifier     mnMainThreadId = 0;
     rtl::Reference< vcl::DisplayConnectionDispatch > mxDisplayConnection;
 
     css::uno::Reference< css::lang::XComponent > mxAccessBridge;

vcl/source/app/svapp.cxx

@@ -520,7 +520,7 @@ comphelper::SolarMutex& Application::GetSolarMutex()
 
 bool Application::IsMainThread()
 {
-    return ImplGetSVData()->mnMainThreadId == osl::Thread::getCurrentIdentifier();
+    return ImplGetSVData()->mpDefInst->IsMainThread();
 }
 
 sal_uInt32 Application::ReleaseSolarMutex()

vcl/source/app/svmain.cxx

@@ -302,9 +302,6 @@ bool InitVCL()
 
     ImplSVData* pSVData = ImplGetSVData();
 
-    // remember Main-Thread-Id
-    pSVData->mnMainThreadId = ::osl::Thread::getCurrentIdentifier();
-
     // Initialize Sal
     pSVData->mpDefInst = CreateSalInstance();
     if ( !pSVData->mpDefInst )