For details see comments and FreeType docs @
https://freetype.org/freetype2/docs/reference/ft2-library_setup.html#ft_library
https://freetype.org/freetype2/docs/reference/ft2-face_creation.html#ft_face

tl;dr: FT_New_Face and FT_Done_Face require the Library to be locked for
thread safety, and FT_Load_Glyph and FT_Render_Glyph and friends need
the face to be locked for thread safety, since we're sharing faces
across threads.

This continues our work to improve the amount of explicit error sets
we use in the codebase. Explicit error sets make it easier to understand
possible failure scenarios, allow us to use exhaustive matching, create
compiler errors if errors are unexpectedly added or removed, etc.

The goal eventually is 100% coverage but we're not even close yet.
This just moves us a little closer.