[samipuro]

Fixes #130.

A large inline image (<img src="data:image/png;base64,...">) inside an element converted with a lazy pattern, such as <li>(.*?)</li> or <a ...>(.*?)</a>, exhausts the PCRE backtrack limit. preg_replace() then returns null, and getText() silently returns an empty string, with only deprecation notices from preg_replace_callback() and strip_tags().

The image source is never part of the text output, so doConvert() now empties data: URIs in img src attributes before the conversion. It uses a possessive pattern, so this step itself runs in linear time. Alt text is still rendered as before.

DataUriTest covers a 1.3 MB image in a list item, in a link and in a paragraph. The list item and link cases return an empty string on master.