Add a special background to use for performance testing

Performance testing was producing inconsistent values at different
times in the day since the GNOME default background is animated and
sometimes has a single layer, and sometimes two blended layers.

So we have consistent numbers, install a simple animated background
with GNOME Shell that has 40-year long transition ending in 2030,a
and set an environment variable in gnome-shell-perf-tool so that the
background is override with that background. (The background depends
on files installed by gnome-backgrounds; we assume that the person
running performance tests is doing so within the scope of a full
GNOME install.)

https://bugzilla.gnome.org/show_bug.cgi?id=734610