How to use loadImage with a GIF?

I have `giflib` installed on my mac. I'm not sure how to use GIF with the library, it doesn't log any error, GIF is not being served in the browser.

Steps to Reproduce
```
app.get('/media', function(req, res) {

const { createCanvas, loadImage, giflib } = require('canvas')

const canvas = createCanvas(600, 400)
const ctx = canvas.getContext('2d')

const gifPath = loadImage('https://www.hubspot.com/hubfs/Smiling%20Leo%20Perfect%20GIF.gif')

gifPath.then((image) => {
// do something with image

canvas.toBuffer((err2, buf) => {
if (err2) {
console.log("To Buffer Error>>>>>>", err2)
}
else
{
res.writeHead(200, {'Content-Type': 'image/gif'});
res.end(buf, 'binary');
}

}, 'image/gif', { quality: 0.95 })

}).catch(err => {
console.log('oh no!', err)
})

})
```

## Your Environment
Canvas Version - canvas@2.8.0
Environment - Mac OS Big Sur 11.1

No repository file or test is named. Start by running the supplied app.get('/media') reproduction with canvas@2.8.0 and inspect the loadImage and canvas.toBuffer path. Done means the GIF URL is handled as intended and the response is served with verified GIF output, or the supported usage and limitation are documented.

Written by the indexing model from the issue text.