Implement copy-on-write for storage/CVT in TrueType hinter to match FreeType fix

[dfrg]

The hinter currently doesn't allocate so this may require some restructuring.

See https://gitlab.freedesktop.org/freetype/freetype/-/merge_requests/23

[drott]

Related FreeType changes and Chromium issue comment:

• https://gitlab.freedesktop.org/freetype/freetype/-/commit/3481b154430faea9f6b4002519e60a8ad8ebe8d8
• https://bugs.chromium.org/p/chromium/issues/detail?id=1190525#c14 and https://gitlab.freedesktop.org/freetype/freetype/-/commit/369d8be97f537c56f2dbd1d3bd23aba74dc24cea