[base] Reintroduce `FT_SqrtFixed`.

The general square root calculations are not necessary in FreeType.
For vector normalization or length, FreeType uses special functions.
It is, however, required in the legacy CFF specifications.

* src/base/ftcalc.c (FT_SqrtFixed): New function that uses either
Babylonian or bit-wise algorithm, whichever is faster for the given
situation.
* include/freetype/internal/ftcalc.h (FT_SqrtFixed): Declare it.