[base] Round values in `FT_MulAdd_Fix`.

This avoids regressing Blink layout tests and makes `FT_MulAdd_Fix` delta
retrieval implementation consistent with the previous implementation, which
used `FT_fixedToInt` and included rounding.

* src/base/ftcalc.c (FT_MulAdd_Fix): Implement it.
Also fix remaining `temp` initialization compilation issue.

Fixes #1169.
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