set clipCircle precision to 2 degrees (#282)

* default geoCircle precision to 2 degrees

Results in 180 control points instead of 60 (⨉3). The error is quadratic with the precision, with the worst case (for a parallel) being at latitude 45°. Under this new setting, the maximum error is now ~484m, vs. 4.365km with precision 6°.
https://observablehq.com/@d3/geocircle-error

* document

* refine clipCircle precision, from 6 degrees to 2 degrees
following #281

(note that for clipCircle the precision is not modifiable by the user)

Manually rechecked each of the tests that are (slightly) modified as a consequence

---------

Co-authored-by: Mike Bostock <mbostock@gmail.com>

src/clip/circle.js

@@ -6,7 +6,7 @@ import clip from "./index.js";
 
 export default function(radius) {
   var cr = cos(radius),
-      delta = 6 * radians,
+      delta = 2 * radians,
       smallRadius = cr > 0,
       notHemisphere = abs(cr) > epsilon; // TODO optimise for this common case
 

test/projection/fit-test.js

@@ -40,8 +40,8 @@ it("projection.fitExtent(…) world azimuthalEquidistant", () => {
 it("projection.fitExtent(…) world conicConformal", () => {
   const projection = geoConicConformal().clipAngle(30).parallels([30, 60]).rotate([0, -45]);
   projection.fitExtent([[50, 50], [950, 950]], world);
-  assertInDelta(projection.scale(), 626.111027, 1e-6);
-  assertInDelta(projection.translate(), [444.395951, 410.223799], 1e-6);
+  assertInDelta(projection.scale(), 625.567161, 1e-6);
+  assertInDelta(projection.translate(), [444.206209, 409.910893], 1e-6);
 });
 
 it("projection.fitExtent(…) world conicEqualArea", () => {
@@ -69,7 +69,7 @@ it("projection.fitExtent(…) world gnomonic", () => {
   const projection = geoGnomonic().clipAngle(45);
   projection.fitExtent([[50, 50], [950, 950]], world);
   assertInDelta(projection.scale(), 450.348233, 1e-6);
-  assertInDelta(projection.translate(), [500.115138, 556.522620], 1e-6);
+  assertInDelta(projection.translate(), [500.115138, 556.551304], 1e-6);
 });
 
 it("projection.fitExtent(…) world mercator", () => {