An HTML comment between two words draws a space in the default parser mode #636

An HTML comment between two words draws a space in the default parser mode, but not in HTML_BODY mode. A browser draws no space.

$mpdf->WriteHTML('<p>a<!-- x -->b</p>');                                // draws "a b"
$mpdf->WriteHTML('<p>a<!-- x -->b</p>', HTMLParserMode::HTML_BODY);     // draws "ab"

In the default mode, CssManager::ReadCSS() calls CommentParser::removeHtmlComments(), and that replaces each comment with a space:

$html = preg_replace('/<!--.*?-->/s', ' ', $html);

In HTML_BODY mode that reader does not run, and AdjustHTML() removes comments without leaving anything in their place.

The same space makes <p><!-- x --></p> hold white space, so p:empty (#537, #634) does not match it in the default mode, although it matches in body mode and in a browser.

Replacing the comment with nothing, as a browser does, would make both modes agree. It changes the spacing of documents that put a comment between two words.

Reproduced on gravitypdf at e685435. Part of #520.