Truncate alt text from src if it's too long.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

TODO

@@ -11,18 +11,11 @@ If no interest is expressed for a feature that may require a considerable
 amount of effort to implement, it may get endlessly delayed. Do not be
 afraid to cast your vote for the next feature to be implemented!
 
-Standing patches:
-
- - Incorporate download and resize support as implemented here:
-   http://htmlpurifier.org/phorum/read.php?3,2795,3628
-
 Things to do as soon as possible:
 
- - Fix ImgRequired to handle data correctly
  - Think about allowing explicit order of operations hooks for transforms
  - Inputs don't do the right thing with submit
  - Fix "<.<" bug (trailing < is removed if not EOD)
- - http://htmlpurifier.org/phorum/read.php?5,2267,4308#msg-4308
  - Build in better internal state dumps and debugging tools for remote
    debugging
 

library/HTMLPurifier/AttrTransform/ImgRequired.php

@@ -24,7 +24,8 @@ class HTMLPurifier_AttrTransform_ImgRequired extends HTMLPurifier_AttrTransform
             if ($src) {
                 $alt = $config->get('Attr.DefaultImageAlt');
                 if ($alt === null) {
-                    $attr['alt'] = basename($attr['src']);
+                    // truncate if the alt is too long
+                    $attr['alt'] = substr(basename($attr['src']),0,40);
                 } else {
                     $attr['alt'] = $alt;
                 }