gdk-pixbuf: use host loaders to update the cache

gdk-pixbuf: use host loaders to update the cache

Unfortunately gdk-pixbuf-query-loaders doesn't understand cross loaders
to update the cache, hence we can't use the host variant against target
loaders since it will output an effectively empty cache, causing runtime
failure of libgtk when finding icons.
So make host-gdk-pixbuf functionally equivalent to the target gdk-pixbuf
so we can run gdk-pixbuf-query-loaders against the host plugins and just
strip the host directory to make it runtime-compatible (like was done
before for the target directory).

This is still better than trying to update at runtime, since that would
require a writable loaders.cache file in tmpfs or rw filesystem, not to
mention the associated additional startup time.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>