C: Make hb_buffer_append_char thread-safe

[marcoroth]

This pull request replaces the static temporary buffer in hb_buffer_append_char with a stack-allocated buffer so parallel calls no longer share mutable state and could corrupt appended output.

Before, it corrupted data like this on rare occasions:

  left: "<piv>\n                            \n    <p>                </p>\n           \n</div>"
 right: "<div>\n                            \n    <p>                </p>\n           \n</div>"

This was only noticeable in the Cargo tests, since they run so fast in parallel and quick succession that it could happen occasionally.