passing a buffer object to `canvas.toBuffer` to reduce gc thrash

`toBuffer` is very handy for blitting a canvas into some other graphics system:
```javascript
const buffer = canvas.toBuffer('raw')
// blit buffer here
```

It would be really nice to pass in an existing buffer as an optional parameter, so it could be re-filled on each `toBuffer` invocation.
That would cut down on gc pressure quite a bit, especially in a tight render loop.