Better polygon intersection test

(see also my comment on 83030be)

js/id/geo.js

@@ -180,9 +180,20 @@ iD.geo.polygonContainsPolygon = function(outer, inner) {
 };
 
 iD.geo.polygonIntersectsPolygon = function(outer, inner) {
+    function testSegments(outer, inner) {
+        for (var i = 0; i < outer.length - 1; i++) {
+            for (var j = 0; j < inner.length - 1; j++) {
+                var a = [ outer[i], outer[i+1] ],
+                    b = [ inner[j], inner[j+1] ];
+                if (iD.geo.lineIntersection(a, b)) return true;
+            }
+        }
+        return false;
+    }
+
     return _.some(inner, function(point) {
         return iD.geo.pointInPolygon(point, outer);
-    });
+    }) || testSegments(outer, inner);
 };
 
 iD.geo.pathLength = function(path) {

test/spec/geo.js

@@ -294,12 +294,18 @@ describe('iD.geo', function() {
             expect(iD.geo.polygonIntersectsPolygon(outer, inner)).to.be.true;
         });
 
-        it('says a polygon that partially intersects does', function() {
+        it('says a polygon that partially intersects does when some points are contained', function() {
             var outer = [[0, 0], [0, 3], [3, 3], [3, 0], [0, 0]];
             var inner = [[-1, -1], [1, 2], [2, 2], [2, 1], [1, 1]];
             expect(iD.geo.polygonIntersectsPolygon(outer, inner)).to.be.true;
         });
 
+        it('says a polygon that partially intersects does when no points are contained', function() {
+            var outer = [[0, 0], [0, 3], [3, 3], [3, 0], [0, 0]];
+            var inner = [[1, -1], [1, 4], [2, 4], [2, -1], [1, -1]];
+            expect(iD.geo.polygonIntersectsPolygon(outer, inner)).to.be.true;
+        });
+
         it('says totally disjoint polygons do not intersect', function() {
             var outer = [[0, 0], [0, 3], [3, 3], [3, 0], [0, 0]];
             var inner = [[-1, -1], [-1, -2], [-2, -2], [-2, -1], [-1, -1]];