Use more stable form of Haversine formula. (#5935)

* Use more stable form of Haversine formula.

The new form does not give a non-zero distance from a point to itself.

* Pre-compute sines which are used twice.

src/geo/crs/CRS.Earth.js

@@ -24,9 +24,10 @@ export var Earth = Util.extend({}, CRS, {
 		var rad = Math.PI / 180,
 		    lat1 = latlng1.lat * rad,
 		    lat2 = latlng2.lat * rad,
-		    a = Math.sin(lat1) * Math.sin(lat2) +
-		        Math.cos(lat1) * Math.cos(lat2) * Math.cos((latlng2.lng - latlng1.lng) * rad);
-
-		return this.R * Math.acos(Math.min(a, 1));
+		    sinDLat = Math.sin((latlng2.lat - latlng1.lat) * rad / 2),
+		    sinDLon = Math.sin((latlng2.lng - latlng1.lng) * rad / 2),
+		    a = sinDLat * sinDLat + Math.cos(lat1) * Math.cos(lat2) * sinDLon * sinDLon,
+		    c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
+		return this.R * c;
 	}
 });