Updates the scroll offset particle.
function _setParticle(position, velocity, phase) {
        if (position !== undefined) {
            //var oldPosition = this._scroll.particle.getPosition1D();
            this._scroll.particleValue = position;
            this._scroll.particle.setPosition1D(position);
            //_log.call(this, 'setParticle.position: ', position, ' (old: ', oldPosition, ', delta: ', position - oldPosition, ', phase: ', phase, ')');
            if (this._scroll.springValue !== undefined) {
                this._scroll.pe.wake();
            }
        }
        if (velocity !== undefined) {
            var oldVelocity = this._scroll.particle.getVelocity1D();
            if (oldVelocity !== velocity) {
                this._scroll.particle.setVelocity1D(velocity);
                //_log.call(this, 'setParticle.velocity: ', velocity, ' (old: ', oldVelocity, ', delta: ', velocity - oldVelocity, ', phase: ', phase, ')');
            }
        }
    }