Rearranged the the optimizations for special cases to reduce the average number of branches.
Rearranged the the optimizations for special cases to reduce the average
number of branches.

Use a non-bogus magic constant for the threshold of pi/4.  It was 2 ulps
smaller than pi/4 rounded down, but its value is not critical so it should
be the result of natural rounding.  Use "<=" comparisons with rounded-
down thresholds for all small multiples of pi/4.

Cleaned up previous commit:
- use static const variables instead of expressions for multiples of pi/2
  to ensure that they are evaluated at compile time.  gcc currently
  evaluates them at compile time but C99 compilers are not required
  to do so.  We want compile time evaluation for optimization and don't
  care about side effects.
- use M_PI_2 instead of a magic constant for pi/2.  We need magic constants
  related to pi/2 elsewhere but not here since we just want pi/2 rounded
  to double and even prefer it to be rounded in the default rounding mode.
  We can depend on the cmpiler being C99ish enough to round M_PI_2 correctly
  just as much as we depended on it handling hex constants correctly.  This
  also fixes a harmless rounding error in the hex constant.
- keep using expressions n*<value for pi/2> in the initializers for the
  static const variables.  2*M_PI_2 and 4*M_PI_2 are obviously rounded in
  the same way as the corresponding infinite precision expressions for
  multiples of pi/2, and 3*M_PI_2 happens to be rounded like this, so we
  don't need magic constants for the multiples.
- fixed and/or updated some comments.

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