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Don't assume XINT result fits in int, or that XFLOAT_DATA * 10
fits in int, when using sprintf.  Use single snprintf to count
length of string rather than counting it via multiple sprintfs;
that's simpler and more reliable.
(APPEND_SPRINTF): New macro.
(font_unparse_fcname): Use it to avoid sprintf buffer overrun.
(generate_otf_features) [0 && HAVE_LIBOTF]: Use esprintf, not
sprintf, in case result does not fit in int.