Alpha channel glitchy when using webp.Decode

Hi.

I'm trying to read some webp files and creating an alpha composite using Draw method with Over as a parameter (code below).
The output shows glitchy lines where the alpha channel is merged:


This code was used:

```go
package main

import (
"fmt"
"image"
"image/draw"
"image/png"
"log"
"os"
"time"

// swap between these two. The glitch is only in this lib
//"golang.org/x/image/webp"
"github.com/chai2010/webp"
)

func getTile(path string) image.Image {
tileStream, _ := os.Open(path)
tile, _ := webp.Decode(tileStream)
defer tileStream.Close()
return tile
}

func main() {
tileA := getTile("a.webp")
tileB := getTile("b.webp")

canvas := image.NewRGBA(image.Rect(0, 0, 500, 500))

draw.Draw(canvas, image.Rect(0, 0, tileA.Bounds().Max.X, tileA.Bounds().Max.Y), tileA, image.ZP, draw.Src)
draw.Draw(canvas, image.Rect(0, 75, tileB.Bounds().Max.X, 75+tileB.Bounds().Max.Y), tileB, image.ZP, draw.Over)

outputStream, _ := os.Create("result.png")
png.Encode(outputStream, canvas)

defer outputStream.Close()
}
```

Using `golang.org/x/image/webp` fixes the issue, but it has a couple other problems therefore I can't use it.


The source files are in the following zip:
[experiments.zip](https://github.com/chai2010/webp/files/4908117/experiments.zip)

Start with the Go reproducer in the issue and the a.webp, b.webp files from experiments.zip. Compare the output from this package with golang.org/x/image/webp, focusing on the alpha data used by draw.Draw with draw.Over. Done means decoding the supplied files produces correct alpha compositing without the visible glitch lines.

Written by the indexing model from the issue text.