When resampling is turned off, d3_geo_resample can return a special
transform that only performs projection of points to 2D, rather than
performing any additional computations such as conversion to 3D
coordinates for resampling.

We we’re distinguishing between the case where the cell completely surrounds the
clip extent and where the cell does not intersect the clip extent at all.
Related #1578.

Also, add a number of missing imports: mainly "math/abs".

Fixes #1581.

If data is updated so that some nodes no longer have children, and a
custom children accessor is used, then the children array was not being
removed for such nodes, which meant that hierarchical layouts were
incorrectly taking into account these children.

Fixes #1579.

This appears to cause a significant performance regression as shown in the
rotating Ocean example (http://bl.ocks.org/mbostock/6713736): performance went
from 60 FPS to 20 FPS.

Previously it was using Date.now(), which might be a millisecond or two past the
now time of the current frame, and thus might not run until the next frame! By
using the current time, we guarantee that it runs at the end of this frame.

Rather than interleave the initialization of transition tweens with their first
tick, defer the first tick to the end of the current timer frame such that any
DOM modifications that happen during the first tick do not trigger a forced
relayout when another tween is initialized (e.g., getComputedStyle).

This makes d3.geom.voronoi consistent with d3.geom.polygon and older versions.

The d3.merge operator must always return a new array, but it wasn’t in the case
where a one-element array was passed. This implementation is still quite fast
but always returns a new array.

Conflicts:
	src/geo/clip-antimeridian.js

Fixes #1533 and avoids “wrong document” error when appending elements to a
different document than the one in which D3 was initialized.

d3.geom.voronoi now returns clockwise polygons instead of
counter-clockwise.  This is consistent with d3.geo’s use of a clockwise
winding order to denote the inside of polygons on the sphere (and
subsequently when projected to 2D).