Adds a new Image Quality Assessment plugin.

Adds a new Image Quality Assessment plugin.

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