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).

Only generate a triangle if the current site is has the minimum index.
This ensures only unique triangles are generated.

Also, ensure triangles are only generated for connected edges, which is
not always the case in unclosed (border) sites.

Turns out, o.x is much faster than o[0]!