gizma.com easing function correction wrapper

- (double)reversibleEaseInOut:(double)t b:(double)b c:(double)c d:(double)d {
    // The alotted time has passed, stop doing weird maths:
    if (t>d) return c;

    // Going the "wrong" direction, needs to be fixed
    BOOL reverse = NO;
    if (c < b) {
        reverse = YES;
        double newc = b;
        b = c;
        c = newc;
    }

    double result = [self easeInOut:t b:b c:c d:d];

    if (reverse) result = c - result;

    return result;
}


// Source: http://gizma.com/easing/#quad3
- (double)easeInOut:(double)t b:(double)b c:(double)c d:(double)d {
    t /= d/2;
    if (t < 1) return c/2*t*t + b;
    t--;
    return -c/2 * (t*(t-2) - 1) + b;
}