[3.0.0] [BACKPORT] More work for hire from Chris

! Experimental support for some proprietary CSS attributes allowed: opacity (and all of the browser-specific equivalents) and scrollbar colors. Enable by setting %CSS.Proprietary to true.
- Colors missing # but in hex form will be corrected
- CSS Number algorithm improved
. New classes:
  + HTMLPurifier_AttrDef_CSS_AlphaValue
  + HTMLPurifier_AttrDef_CSS_Filter

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1473 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/AttrDef/CSS/AlphaValue.php

@@ -0,0 +1,22 @@
+<?php
+
+require_once 'HTMLPurifier/AttrDef.php';
+require_once 'HTMLPurifier/AttrDef/CSS/Number.php';
+
+class HTMLPurifier_AttrDef_CSS_AlphaValue extends HTMLPurifier_AttrDef_CSS_Number
+{
+    
+    public function __construct() {
+        parent::__construct(false); // opacity is non-negative, but we will clamp it
+    }
+    
+    public function validate($number, $config, &$context) {
+        $result = parent::validate($number, $config, $context);
+        if ($result === false) return $result;
+        $float = (float) $result;
+        if ($float < 0.0) $result = '0';
+        if ($float > 1.0) $result = '1';
+        return $result;
+    }
+    
+}