Better boolean operations for V3

[HarryStevens]

The current relationship methods return only booleans, i.e. if the relationship exists. Better would be to return where the relationship exists, or false if it does not exist.

• lineIntersectsLine: https://observablehq.com/d/6222f0815840b5e4
• lineIntersectsPolygon: https://observablehq.com/d/efb47e989a0bbc9c

The polygon relationship methods will likely be somewhat time consuming to implement. The most modern algorithms for performing boolean operations on polygons can be found here: http://www.cs.ucr.edu/~vbz/cs230papers/martinez_boolean.pdf

These will likely not be compatible with current methods, so it may be necessary to bump major versions.

[HarryStevens]

It will likely also be necessary to implement a utility method called, say, polygonWind, polygonSort, or polygonOrder because the Martinez et al clipping algorithms require a polygon's vertices to be in counter-clockwise order.

Here's an implementation: https://observablehq.com/d/eaf3a3c863d34aee

[HarryStevens]

See also: http://sean.cm/a/polygon-clipping-pt1

[HarryStevens]

On simple polygons, see: https://en.wikipedia.org/wiki/Simple_polygon

[HarryStevens]

Fil's done it: https://observablehq.com/@d3/polygonclip

[ianstormtaylor]

Would love to see this added to the library!