make drawing area transparent by default

Change-Id: Ib31d68ce13b74f3f92e81e09bef7eaed2d7f9708
Reviewed-on: https://gerrit.libreoffice.org/55862
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

include/vcl/layout.hxx

@@ -702,6 +702,7 @@ public:
         , m_pFactoryFunction(nullptr)
         , m_pUserData(nullptr)
     {
+        SetBackground();
     }
     void SetUITestFactory(FactoryFunction pFactoryFunction, void* pUserData)
     {