There is a significant performance issue in get_unihash(). The issue turns out
to be the lookups of setscene tasks. We can fix this by using a set() instead of
the current list.
(Bitbake rev: 1e561672d039ebfb8cd0e0654a44dcf48513317c)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>