tdf#94006 - need an explicit dispose for slideshow's GLContext.

Fixes crasher exiting slideshow.

Change-Id: I14b6a2c1abc946f590bf91cab19099753172fd95

slideshow/source/engine/OGLTrans/generic/OGLTrans_TransitionerImpl.cxx

@@ -1325,6 +1325,8 @@ void OGLTransitionerImpl::impl_dispose()
 {
     impl_finishTransition();
     disposeTextures();
+    if( mpContext.is() )
+        mpContext->dispose();
     mpContext.clear();
 }