Log Message:
harfbuzz: update to 5.1.0.

Overview of changes leading to 5.1.0
Sunday, July 31, 2022
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- More extensive buffer tracing messages. (Behdad Esfahbod)
- Fix hb-ft regression in bitmap fonts rendering. (Behdad Esfahbod)
- Support extension promotion of lookups in hb-subset-repacker. (Garret Rieger)
- A new HB_GLYPH_FLAG_SAFE_TO_INSERT_TATWEEL for scripts that use elongation
  (e.g. Arabic) to signify where it is safe to insert tatweel glyph without
  interrupting shaping. (Behdad Esfahbod)
- Add “--safe-to-insert-tatweel” to “hb-shape” tool. (Behdad Esfahbod)

- New API
+HB_GLYPH_FLAG_SAFE_TO_INSERT_TATWEEL
+HB_BUFFER_FLAG_PRODUCE_SAFE_TO_INSERT_TATWEEL