lanczos = function lanczos(x, a) {

        // Lanczos is sinc(x) * sinc(x / a) for -a < x < a ...
        if (-a < x && x < a)
            return sinc(x) * sinc(x / a);

        // ... and 0 otherwise
        return 0;

    }
Calculates the value of the Lanczos kernal at point x for a given window
size. See: https://en.wikipedia.org/wiki/Lanczos_resampling

@private
@param {Number} x
    The point at which the value of the Lanczos kernel should be
    computed.

@param {Number} a
    The window size to use for the Lanczos kernel.

@returns {Number}
    The value of the Lanczos kernel at the given point for the given
    window size.