tests: Fill the default pixbuf with the placeholder colour
tests: Fill the default pixbuf with the placeholder colour

Ideally, we should only be comparing data that the loader told us it
updated, and this is kind of what we're doing, as long as we don't have
a legitimate image that uses the exact same filler colour. Unlikely
though.

https://bugzilla.gnome.org/show_bug.cgi?id=696331

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf