Fix --toc-depth with beamer output.

Previously only top-level sections were ever included in the TOC,
regardless of the setting of `--toc-depth`.

Closes #9861.

Author: jgm

data/templates/default.latex

@@ -571,7 +571,8 @@ $if(beamer)$
 $if(toc-title)$
   \frametitle{$toc-title$}
 $endif$
-  \tableofcontents[hideallsubsections]
+  \setcounter{tocdepth}{$toc-depth$}
+  \tableofcontents
 \end{frame}
 $else$
 {