Avoid Microsoft compiler warnings (#51331).

While clang's sanitizer recommends a cast to unsigned for safe
negation (to handle -INT_MIN), both MSVC and Visualc emit warning
C4146 if an unsigned value gets negated.

* include/freetype/internal/ftcalc.h (NEG_LONG, NEG_INT32),
src/base/ftcalc.c (FT_MOVE_SIGN): Replace negation with a
subtraction.
3 files changed