Better region label positioning

[ryan-williams]

The "Examples" section shows instances where the labels are placed on top of one another, outside the region they are meant to label, etc. For example:

(see the TP53 "182" vs. $\text{TP53} \cap \text{STK11}$'s "23")

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Each region's label position is currently an averaging of its edges' mid- and quarter-points. Ideally it would be the point in the region that is furthest from any edge.