fix some buglets in surrogate pair decoding

First, the correct constant to use is 0x10000, aka Math.pow(2,20).
Second, arrange to skip the low surrogate.  Finally, fix the math in
stringFromCode as well, and remove an extraneous semicolon.

Author: tromey

parse-css.js

@@ -47,7 +47,8 @@ function preprocess(str) {
 			// Decode a surrogate pair into an astral codepoint.
 			var lead = code - 0xd800;
 			var trail = str.charCodeAt(i+1) - 0xdc00;
-			code = Math.pow(2, 21) + lead * Math.pow(2, 10) + trail;
+			code = Math.pow(2, 20) + lead * Math.pow(2, 10) + trail;
+			i++;
 		}
 		codepoints.push(code);
 	}
@@ -57,9 +58,9 @@ function preprocess(str) {
 function stringFromCode(code) {
 	if(code <= 0xffff) return String.fromCharCode(code);
 	// Otherwise, encode astral char as surrogate pair.
-	code -= Math.pow(2, 21);
+	code -= Math.pow(2, 20);
 	var lead = Math.floor(code/Math.pow(2, 10)) + 0xd800;
-	var trail = code % Math.pow(2, 10); + 0xdc00;
+	var trail = code % Math.pow(2, 10) + 0xdc00;
 	return String.fromCharCode(lead) + String.fromCharCode(trail);
 }