This doubles the number or allowed points, see
    https://github.com/harfbuzz/harfbuzz/issues/4752

Although it is hardly practical to use more than 32767 points,
other font engines seem to support it.

* docs/CHANGES: Announce it.
* include/freetype/ftimage.h (FT_Outline): Do it and update limits.
* src/*: Update `FT_Outline` users.