There are four significant constants set in globals.h
At the time of writing there still is no standard consistent across platforms so
please use these in all new calculations.

#define PI 3.1415926536f
#define LOG_2 0.693147181f

The above were always defined right back to Zyn 2.2.1
We've kept the values the same to ensure consistency.

#define TWOPI 6.28318530718f
#define HALFPI 1.57079632679f

These were introduced in Yoshimi 1.2.2
Throughout Yoshimi these had been computed in different ways, and it was thought
this, along with unknown compiler optimisations could give varying results.
Typically:
2 * {some calculation} * PI
{some calculation} * 2 * PI
PI * 2.0f

{some calculation} * PI / 2
PI * {some calculation} / 2.0f
