对比cwebp命令行，带透明度png图片转换失真明显

使用下面代码进行png转webp

```
func ImgTowebp(imgData []byte) ([]byte, error) {
//jpeg.Decode()
img, err := jpeg.Decode(bytes.NewBuffer(imgData))
if err != nil {
//log.Println("decode error from jpg",err)
img, err = png.Decode(bytes.NewBuffer(imgData))
if err != nil {
log.Println("decode error from png or jpg", err)
return nil, err
}
}
//log.Println(fname)
webImgBytes, err := webp.EncodeRGBA(img, 80)
if err == nil {
webpLength := len(webImgBytes)
imgLength := len(imgData)
if webpLength > imgLength {
log.Println("webp bigger than img")
}
log.Println("img2webp :", webpLength, ":", imgLength)
return webImgBytes, nil
} else {
log.Println("webp encode jpg file", err)
return nil, err
}

}

```
对比用cwebp命令行
```
cwebp -q 80 test.png -o test.png.webp
```

发现图片的颜色变化明显，排查源代码没有发现明显错误

测试图片：


Start with the ImgTowebp example in the issue and reproduce its output using the supplied transparent PNG. Compare that output with cwebp -q 80 and trace the PNG decoding and WebP encoding path. Done means transparent PNG colors no longer differ noticeably from the cwebp result, with a regression test if the project provides a suitable test location.

Written by the indexing model from the issue text.