xiph_metadata: Don't always force UTF8 encoding

When profiling the media library on a small samples folder, the calls to
EnsureUTF8 end up accounting for about 8% of the runtime (including
calls to vlc_towc)
By not always executing it, especially when the value is expected *not*
to be UTF8 but ascii, we end up spending only 0.04% of the time in
EnsureUTF8
This also saves useless calls when the value is to be dropped