Polygon shrinking

[maxnth]

Add various polygon shrinking mechanisms like:

• Shrinking based on hull of child elements
• Shrinking based on connected components
• etc.

[bertsky]

BTW, there are implementations for this in various OCR-D processors already. It might help having a look:

• Shrinking based on hull of child elements

see ocrd-segment-project's join_polygons, which uses Shapely to find the nearest points between each neighbouring polygon and then connects them with a straight line expanded into a rectangle (after finding the shortest path through all polygons efficiently).

• Shrinking based on connected components

see ocrd-segment-repair's sanitize_region, which uses OpenCV's connectedComponents and findContours