Export proper navigation images

[lanery]

The small.png images created during export are off. Found some documentation about how to make them properly
https://catmaid.readthedocs.io/en/stable/user_faq.html#how-to-create-the-small-overview-images-in-the-lower-right-corner

[lanery]

Can get this to work by creating a bbox image

thumb = icatapi.render_bbox_image(stack=stack,
                                  z=z,
                                  bbox=bbox,
                                  width=192,
                                  maxTileSpecsToRender=1000,
                                  render=render)

where the bbox is defined by the extreme limits of all the stacks to export. This is fine except will take a long time to render hi-mag EM image overview images for multiple sections. Would be better to create from highest zoom level images, but then sometimes the max zoom level doesn't exist for hi-mag EM (because the render_catmaid_boxes script doesn't do this unfortunately. So would have to manually create the max zoom level image from the 4 max-1 zoom level images (which is arguably nice to do anyway) and then properly create the small.png navigation image from this.