Don’t do surrogate replacement when preprocessing input stream

Fixes #383

This change drops the code for replacing surrogate code points from our
implementation of “filter code points” from “Preprocessing the input
stream” at https://drafts.csswg.org/css-syntax/#css-filter-code-points

w3c/csswg-drafts#3307 (comment)
notes that the only way to produce a surrogate code point in CSS content
is by directly assigning a DOMString with one in it via an OM operation;
in other words, by manipulating a document using JavaScript to insert
a surrogate code point into the document.

But because the CSS validator doesn’t execute any JavaScript from a
document, there’s no way for a document being checked by the CSS
validator to contain any surrogate code points. Therefore, it’s
unnecessary for our implementation to handle replacement of surrogate
code points. In other words, our implementation can still conform to the
spec requirements even if we don’t perform surrogate replacement.

Author: sideshowbarker

org/w3c/css/util/UnescapeFilterReader.java

@@ -31,9 +31,6 @@ public int read()
         if (c == 0) { // U+0000 NULL
             return 0xfffd; // U+FFFD REPLACEMENT CHARACTER
         }
-        if (c >= 0xd800 && c <= 0xdfff) { // surrogate
-            return 0xfffd;
-        }
 
         // now specific case of CSS unicode escape for ascii values [A-Za-z0-9].
         if (c != '\\') {
@@ -108,8 +105,6 @@ public int read(char[] cbuf, int off, int len) throws IOException {
                 chars[j++] = 0x000a;
             } else if (chars[i] == 0) {
                 chars[j++] = 0xfffd;
-            } else if (chars[i] >= 0xd800 && chars[i] <= 0xdfff) {
-                chars[j++] = 0xfffd;
             }
             // escaping