update setSource in nextTick

[Gcaufy]

When set img.src=some_buffer the onload callback function may not be called.

Demo code:

const img = new Image();
img.src = some_image_buffer;
img.onload = () => { alert('success'); }

In this case, onload will never be called.
After I check the code below:
https://github.com/Automattic/node-canvas/blob/master/src/Image.cc#L278-L281

I change the code to:

const img = new Image();
img.onload = () => { alert('success'); }
img.src = some_image_buffer;

And this code works well.

• Have you updated CHANGELOG.md?
  No, I will update it after the pr was approved.

[jimmywarting]

if createImageBitmap(blob) is being introduced, then new Image() will be discouraged, depricated or removed