If that threshold is reached, `iqa` will emit an ERROR message on the
bus, stopping any processing.
This way we can do a simpler comparison with gst-validate and the
process will error out if the specified threshold is reached.
https://bugzilla.gnome.org/show_bug.cgi?id=795428
It only offers one metric for now, "dssim", available if
https://github.com/pornel/dssim was installed on the system
at the time the plugin was compiled.
The spearman correlation for dssim against the TID2008 dataset
is 0.81, against 0.70 for the old ssim implementation, and
it runs 15 times faster.
https://bugzilla.gnome.org/show_bug.cgi?id=751324
