Since 3.1, the Mercator projection does not clamp y-values. For rendering
latitudes near the pole, the y-values can approach infinity. Rather than
clamping each point in the projection, d3.geo.mercator now sets the clipExtent
automatically (if not overridden) using the projection's scale and translate.

Also, drop *this* context in all cases.

I blame @mbostock. :)

Also, limit size to [width, height].

See #1145.

See #1145.

A slight correction to clipLine fixed the problem with points lying
exactly on the clip edge, but in case clipLine(a, b) ever returns false
(no intersections) even though v_=0 and v=1, we treat this as the start
of a line anyway.