gradient color is 'transparent' by browser is inconsistent with that drawn by server.

## Issue
browser is inconsistent with that drawn by server.
if change transparent to `#00000000`，it works


It looks like a white background covers the gradient background
## Steps to Reproduce
```js
const { createCanvas } = require('canvas')
const canvas = createCanvas(1080, 1920)
const ctx = canvas.getContext('2d')

var grd=ctx.createLinearGradient(0, 960, 1080, 960);
grd.addColorStop(0,"transparent");
grd.addColorStop(1,"#ffffff");

ctx.fillStyle = '#fff'
ctx.fillRect(0, 0, 1080, 1920)

ctx.fillStyle=grd;
ctx.fillRect(0, 0, 1080, 1920);

const fs = require('fs')
const out = fs.createWriteStream(__dirname + '/test.png')
const stream = canvas.createPNGStream()
stream.pipe(out)
```

## Your Environment
* node-canvas 2.6.1
* Environment (node v14.15.2 on Mac OS X 10.15.6):