Unusual principal value for cLog

[pjt33]

cLog is implemented as

vec2 cLog(vec2 a) {
    float b =  atan(a.y,a.x);
    if (b>0.0) b-=2.0*3.1415;
    return vec2(log(length(a)),b);
}

The resulting imaginary part is rather unexpected: the two main conventions for the principal value of arctan are that it runs from -pi to pi or from 0 to 2pi, not (as here) from -2pi to 0.

I presume that it's implemented as present because the Ducks fractal requires an unusual branch cut, but I question whether the implementation with that branch cut should be the "default" one, the only one provided in Complex.frag.

I have three suggestions, and I'll happily write a patch if one of them is acceptable:

1. Provide three different log methods with different names and branch cuts: cLogPos (0 to 2pi), cLog (-pi to pi), cLogNeg (-2pi to 0).
2. Provide an overload which takes an int parameter and switches on it.
3. Add a control uniform int logBranchCut; slider [-1,0,1] and switch on its value.