fcft_precompose - pre-compose two wide characters into one
fcft_precompose() composes the base and comb wide characters into a single wide character. For example, L'a' + L'\u0301' can be combined to L'á'.
If *_is_from_primary are non-NULL, they will be true if the corresponding wide character exists in the primary font. If false, it does not exist in the primary font and fcft_glyph_rasterize(3) will pick the glyph from one of the fallback fonts.
On success, fcft_precompose() returns a composed wide character. On error, (wchar_t)-1 is returned.