itemize: Improve script itemization

Merge marks, and other cluster extenders with the previous script. Logic copied from: https://searchfox.org/mozilla-central/rev/dbc0cd5615f9cd7337d3e05b7c3925af5f35c0ee/gfx/thebes/gfxScriptItemizer.cpp#113-122

Handling of script extensions is still missing as GLib does not seem to have an API to retrieve it.

Fixes #739 (closed)