Smarter rendering of small objects

[andreped]

Similar to https://github.com/AICAN-Research/FAST-Pathology/issues/32, if you zoom out, small objects become invisible. In that scenario it makes sense to merge all together, or similar, if they are close, when rendering are low resolution.

However, for mitoses, which commonly are quite rare in a WSI, you will not have the same effect. Perhaps it could be a good idea to support drawing disks (or similar) of different sizes, such that when you zoom out, you can visualize where the mitoses are located in a WSI.

That would be a great solution for visualizing how the mitoses are distributed in a WSI, and it enables the user to very easily find the hotspot, relevant for measuring the mitosis activity.