Improve layer rendering

[andreped]

When rendering results, for instance for nuceli, the segmentations become quite invisible when zooming out.

I think it might be a good idea to add some type of rendering effect which enables it to visualize segmentations also when zoomed out. For instance if there are lots of nuclei at the same spot, all colored green, if you zoom out, all segmentations should merge into a "larger segmentation", which can be visualized.

Perhaps this is something we could enable/disable, but I think that might look better.

What are your thoughts?