background: Remove more bogus checks

The content in these arrays can never be null...

https://bugzilla.gnome.org/show_bug.cgi?id=719803
background: Don't wait for gdk-pixbuf to fail before loading animations

We don't have any better way of determining whether something is a slideshow
animation, so discriminate on the .xml filename instead of waiting for
gdk-pixbuf to determine whether it can load a file or not.

https://bugzilla.gnome.org/show_bug.cgi?id=719803
background: Remove the system noise content when not in use

https://bugzilla.gnome.org/show_bug.cgi?id=719803
background: Remove unused variable

https://bugzilla.gnome.org/show_bug.cgi?id=719803
background: Simplify animation code

https://bugzilla.gnome.org/show_bug.cgi?id=719803
background: Clarify the intent of the code

Stomping on local variables and trying to keep loop state isn't
too fun. Just use a new variable here so we aren't too confused
with what we're doing.

https://bugzilla.gnome.org/show_bug.cgi?id=719803
background: Add copied content from pending image loads to the cache

https://bugzilla.gnome.org/show_bug.cgi?id=719803
background: Don't silently fizzle out when removing bad content from the cache

If the background is already removed, or we're trying to remove bad content,
this is probably a bug in content accounting, so let us crash so we can fix
the bugs.

https://bugzilla.gnome.org/show_bug.cgi?id=719803