Greiner–Hormann is preferable to Sutherland-Hodgman, because it doesn't
add extraneous lines along the clip edge, and can even handle
self-intersecting polygons.  The line segments are generated via the
efficient Liang–Barsky line clipping algorithm, and Greiner–Hormann
rejoins them appropriately along the clip edge.

Still in progress: correct marking of whether an intersection is an
entry or exit point, and detection of points that lie exactly on the
clip edge, which should be jittered.

Also, remove a ring.pop() from d3_geo_clipView's ringEnd that was causing
missing vertices when testing against a map of U.S. counties. Just a guess!

This ensures that executables are consistent with the local package.json and
not affected by any globally-installed packages.

Conflicts:
	src/geo/interpolate.js

Still need to implement sensible behavior for log.ticks without base 10, but I
haven't decided what would be appropriate yet.

This is a simple stream transformation for the moment.  Note that the
clip region points should be specified in counterclockwise order as
required by d3.geom.polygon, and unlike GeoJSON, the first and last
points should not be coincident.

After intersecting a polygon with a clip region, it's possible for one
or more of the resulting visible segments to be a closed ring.  This
occurs when the first and last points in a segment are coincident (or
close enough to be coincident).

If the clipping algorithm attempts to rejoin these segments, first it
orders intersection points along the clip edge, and the subsequent
interpolation direction depends on this order.  However, the order of
coincident points is ambiguous, hence “flickering” may occur if the
order of two coincident swaps around, causing interpolation to alternate
between taking the longer and shorter ways around the clip edge.

Also, fix vows clobbering our custom asserts.

Seems clumsy to only expose this via projection.rotation, and if we do expose it
by itself, it seems redundant to also expose projection.rotation. So instead we
now have d3.geo.rotation.

A more user-friendly version of d3.geom.voronoi. Fixes #1100.

Similar to selection.select(function), the before argument to selection.insert
can now be specified as a function, so that the caller can specify the before
node dynamically. Supersedes #1071.

Also return the perimeter for polygons.

This is useful if you want to specify the center in non-rotated coordinates.

Fixes #931.