Merge overlapping rect highlights

[robertknight]

If many users draw rect highlights over the same area, this can end up looking messy and obscuring the content:

This could perhaps be fixed by merging highlights whose shapes are sufficiently close. In computer vision a common metric for measuring similarity of bounding boxes is to take the intersection-over-union, so perhaps that could be used. Implementing this would have the consequence that adding or removing a highlight may alter how other highlights are displayed.