May want to resize points based on ROI image size

[vreuter]

Testing has been done on images of (8, 16, 16) = (z, y, x), and with a point size of 0.5 (or 1 at most). Most likely, we'll want to make the point size (and text size) dynamic and based on the ROI image size. To do so, we'll need to pass that information somehow along from the Scala program to the disk representation of the points data and adapt this plugin accordingly, or somehow use napari machinery to get the size of the loaded spot ROI image at runtime.

[vreuter]

Related to resizing in #1 and #2