harfbuzz: Update to 2.7.4

Changelog:
2.7.4
    Fix missing --enable-introspection configure option from previous release tarball.
    Documentation updates.

2.7.3
    Update USE shaper to 2020-08-13 specification, and other improvements.
    Don't disable liga feature in myanmar shaper, to match Uniscribe.
    Improvements to language and script tags handling.
    Update language system tag registry to OpenType 1.8.4
    Support for serializing and deserializing Unicode buffers. Serialized buffers
    are now delimited with <> or [] based on whether it is a Unicode or
    glyphs buffer.
    Increase buffer work limits to handle fonts with many complex lookups.
    Handle more shaping operations in trace output.
    Memory access fixes.
    More OOM fixes.
    Improved documentation.
    Build system improvements.
    New API:
    +hb_buffer_has_positions()
    +hb_buffer_serialize()
    +hb_buffer_serialize_unicode()
    +hb_buffer_deserialize_unicode()

(ryoon)