Local image will not preview in VSCode if code block with markdown output is in document

[jwoLondon]

Thanks to @nickysinclair for bringing this to my attention:

In VSCode, but not Atom, the following Litvis document will not preview the image when stored locally. Examples here use triple-hat notation, but also applies to full code blocks:

^^^elm m=(3+4)^^^
![Does image show?](myImage.png)

If output is changed to raw rather than markdown, there is no problem:

^^^elm r=(3+4)^^^
![Does image show?](myImage.png)

If the image is referenced via a URL to an external CORS-compliant sever, there is no problem:

^^^elm m=(3+4)^^^
![Does image show?](https://gicentre.github.io/data/images/LillyTarn.jpg)

The fact it is VSCode specific, and only to local files suggests it might be related to the cause of #49 although I don't know why it only applies to markdown output.