Utility method loadImage memory leak

## Issue

Calling the utility method `loadImage` (or just using the `Image` object for the matter) does not release the memory used.
The same happens with both local and remote images. Loading a 3MB remote jpg in the below example would keep the rss memory at ~400MB and would not be GCed.

Awaiting for the promise to resolve, setting the returned image to null / the source to null makes no difference to the memory level.

## Steps to Reproduce

```js
const { loadImage } = require('canvas')

for (let i=1; i<=10; i++) {
loadImage('path_to_image').then(function(){
console.log('loaded')
})
}

setInterval(()=>{
if (global.gc) { global.gc() }
console.log('rss: ' + process.memoryUsage().rss / (1024 * 1024) + 'MB')
}, 1000)
```

## Your Environment
* Version of node-canvas: 2.6.1 built from source
* Environment: node 12.19.1 & node 14.15.1 on MacOS 10.13.6 & Heroku 18 stack (Ubuntu 18.04)