We need to include the icon size in the data we pass to on_pixbuf_loaded()
so that we can include it in the key we use when caching the icon
Currently we save icons in the cache at size 0 but look them up at size 48
so no caching occurs.
http://bugzilla.gnome.org/show_bug.cgi?id=583503
Add a ShellTextureCache class which loads (and can cache)
pixmap->texture conversions. This fixes a problem with the
async code in ClutterTexture that it was lower priority
than animations, and also ensures we're really only
loading these pixbufs once in the icon case.