The official Ut Video decoder only threads with slices, thus until
now any files encoded by the libavcodec encoder have only been
decodable with a single thread. The default slice count is now
set to subsampled_height / 120.
Also sets slices to 1 for the Ut Video encoder tests to keep them
green.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>