st-texture-cache: Don't cache GIcons which cannot be serialized

st-texture-cache: Don't cache GIcons which cannot be serialized

For GIcons we use g_icon_to_string() in the key, but the function
will return NULL if the icon cannot be serialized. As a result,
all non-serializable GIcons of the same size end up with the same
cache key - an example for this are contacts with avatars, which
currently all end up with the same image.
To fix, opt out of caching for GIcons which cannot be serialized.

https://bugzilla.gnome.org/show_bug.cgi?id=660585