Compose the characters at FROM by FUNC.
FUNC is called with one argument GSTRING which is built for characters
in the region FROM (inclusive) and TO (exclusive).
If the character are composed on a graphic display, FONT-OBJECT
is a font to use. Otherwise, FONT-OBJECT is nil, and the function
`compose-gstring-for-terminal' is used instead of FUNC.
If STRING is non-nil, it is a string, and FROM and TO are indices
into the string. In that case, compose characters in the string.
The value is a gstring containing information for shaping the characters.
This function is the default value of `auto-composition-function' (which see).