strip_pdf_comments tracked parenthesis nesting to protect string
literals, but ignored backslash escapes. An escaped \) desynced the
depth counter, after which a % glyph inside a string was stripped as a
top-level comment, corrupting the stream for Content::decode and
silently truncating the page's text.
Treat \ inside a string literal as escaping the next byte, so \(,
\), and \\ never touch the nesting depth.
