function premultiplyRgba(rgb, alpha, out, premultiply) {
    out = out || new Float32Array(4);
    if (premultiply || premultiply === undefined) {
        out[0] = rgb[0] * alpha;
        out[1] = rgb[1] * alpha;
        out[2] = rgb[2] * alpha;
    }
    else {
        out[0] = rgb[0];
        out[1] = rgb[1];
        out[2] = rgb[2];
    }
    out[3] = alpha;
    return out;
}
combines rgb and alpha to out array
@memberof PIXI.utils
@function premultiplyRgba
@param {Float32Array|number[]} rgb - input rgb
@param {number} alpha - alpha param
@param {Float32Array} [out] - output
@param {boolean} [premultiply=true] - do premultiply it
@returns {Float32Array} vec4 rgba