distance function
Calculates the euclidian distance between two vec4's
@param {ReadonlyVec4} a the first operand @param {ReadonlyVec4} b the second operand @returns {Number} distance between a and b
Implementation
double distance(List<double> a, List<double> b) {
final x = b[0] - a[0];
final y = b[1] - a[1];
final z = b[2] - a[2];
final w = b[3] - a[3];
return hypot([x, y, z, w]);
}