crash on launching opengl slide transitions under gtk3

since...

commit 9090dc1f3b
Date:   Mon Jan 18 18:27:19 2021 +0200

    split OutputDevice from Window

note SpriteDeviceHelper::getDeviceHandle seems to return a vcl::Window
and not an OutputDevice like the other ::getDeviceHandle so that
probably needs changing too, or the others need to return the owner
window.

Change-Id: I84da379e680d3da5775855a16dc716decef22bbf
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/116052
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Jenkins

slideshow/source/engine/opengl/TransitionerImpl.cxx

@@ -314,9 +314,12 @@ bool OGLTransitionerImpl::initWindowFromSlideShowView( const Reference< presenta
 
     mpContext = OpenGLContext::Create();
 
-    if( !mpContext->init( reinterpret_cast< vcl::Window* >( aVal ) ) ) {
+    OutputDevice* pDevice = reinterpret_cast<OutputDevice*>(aVal);
+    vcl::Window* pWindow = pDevice ? pDevice->GetOwnerWindow() : nullptr;
+
+    if( !mpContext->init( pWindow) ) {
         mpContext->requestLegacyContext();
-        if( !mpContext->init( reinterpret_cast< vcl::Window* >( aVal ) ) )
+        if( !mpContext->init( pWindow ) )
             return false;
     }
     SAL_INFO("slideshow.opengl", "created the context");