Rendered images should use Markdeep class=pixel

[hollasch]

Currently, we're wrapping rendered images in div.render. We don't need to do that, as Markdeep already has a pixel image type built in.

Instead of this:

<div class="render">

![A simple red sphere](../images/img.red-sphere.png)

</div>

we should just use this:

![A simple red sphere](../images/img.red-sphere.png class=pixel)