Scrolls to anchor before GatsbyImage has loaded

[konstantinschuette]

Hello,

Steps to reproduce:

1. Clone my project https://github.com/KonstantinSchuette/gatsby-anchor-bug
2. gatsby develop
3. Fire up a incognito tab and navigate to http://localhost:8000/page-2
4. Then click on GO TO ANCHOR
5. Now you should see this:
   

I think the anchor plugins doesn't respect the height of the image, because the offset is equal to the height of the image.

[konstantinschuette]

This does not happen, when you use Gatsbys built in <Link>.