utvideoenc: Enable support for multiple slices and use them

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: Derek Buitenhuis <derek.buitenhuis@gmail.com>