One of the tinderboxes fails with mysterious warning:

include/tools/gen.hxx:418:36: error: '*((void*)& aViewportExt +16)' may
be used uninitialized in this function [-Werror=maybe-uninitialized]
     nBottom = rSize.Height() ? nTop+rSize.Height()-1 : RECT_EMPTY;

Change-Id: I62bf99d1a623ba4bd9d7db5fe049366d0f20e26f