Fix #1437 - add identity projection.

The identity projection is equivalent to setting path.projection(null), except
that you can apply post-projection features such as viewport clipping.

d3.js

@@ -3932,6 +3932,21 @@ d3 = function() {
   (d3.geo.gnomonic = function() {
     return d3_geo_projection(d3_geo_gnomonic);
   }).raw = d3_geo_gnomonic;
+  d3.geo.identity = function() {
+    var clipExtent = null;
+    function identity(x) {
+      return x;
+    }
+    identity.invert = identity;
+    identity.stream = identity;
+    identity.clipExtent = function(_) {
+      if (!arguments.length) return clipExtent;
+      clipExtent = _;
+      identity.stream = _ == null ? identity : d3_geo_clipView(_[0][0], _[0][1], _[1][0], _[1][1]);
+      return identity;
+    };
+    return identity;
+  };
   function d3_geo_mercator(λ, φ) {
     return [ λ, Math.log(Math.tan(π / 4 + φ / 2)) ];
   }

src/geo/identity.js

@@ -0,0 +1,20 @@
+d3.geo.identity = function() {
+  var clipExtent = null;
+
+  function identity(x) {
+    return x;
+  }
+
+  identity.invert = identity;
+
+  identity.stream = identity;
+
+  identity.clipExtent = function(_) {
+    if (!arguments.length) return clipExtent;
+    clipExtent = _;
+    identity.stream = _ == null ? identity : d3_geo_clipView(_[0][0], _[0][1], _[1][0], _[1][1]);
+    return identity;
+  };
+
+  return identity;
+};

src/geo/index.js

@@ -26,6 +26,7 @@ import "conic-equal-area";
 import "conic-equidistant";
 import "equirectangular";
 import "gnomonic";
+import "identity";
 import "mercator";
 import "orthographic";
 import "stereographic";