[smooth] Avoid integer overflow. Reported as https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=36243 * src/smooth/ftgrays.c(ADD_LONG, SUB_LONG, MUL_LONG, NEG_LONG) [STANDALONE_]: Removed, unused. (ADD_INT) [STANDALONE_]: New macro. (FT_INTEGRATE): Use ADD_INT.