lyluatex produces real numbers that lilypond doesn't understand

[lemzwerg]

As mentioned in issue #268, lyluatex contains a buglet that produces invalid real numbers.

Example:

\documentclass{article}

\usepackage{lyluatex}


\begin{document}

\begin{lilypond}[insert=fullpage,
                 extra-top-margin=-125.27pt,
                 debug]
  c'1
\end{lilypond}

\end{document}

The temporary file now contains

top-margin = -1.5000000004761e-05\pt

which makes lilypond complain:

987e5fb03b54c78dba5c5218ec607f2c.ly:20:35: Fehler: syntax error, unexpected REAL, expecting '.' or '='
    top-margin = -1.5000000004761e
                                  -05\pt