Rework the x264 speedcontrol presets.

Since speedcontrol was made, x264 has adjusted its “slower” preset,
so follow suit. This influences some of the other presets as well.

Unfortunately, I couldn't reproduce my previous results using
“Tears of Steel”, and the original scripts to do so have been lost;
when testing, the SSIM improves way too slowly. Switch to
“Elephants Dream” instead, even though --trellis 1 seems to yield
a slight SSIM _decrease_ for this clip, and check in the new scripts
to increase reproducibility of results. The measurements are on the
same machine. The timings have shifted around a bit (it looks like
the two fastest modes are faster, relatively speaking); potentially
due to x264 optimizations, but more likely due to differing content
and bitrate flags.

This is measured at the x264 version which is in Debian stretch,
which is r2748 (git 97eaef2).