subfigure figure panel breakage

[brucemiller]

It seems that arrange_panels_and_breaks introduces an ltx:block wrapper around things including ltx:figure which is then invalid. Turned up when experimenting with #2708, if I tweak the acmart test by adding actual images to the subfigures:

\documentclass[acmsmall]{acmart}
\usepackage{graphicx}
\usepackage{subfigure}
\begin{document}
\begin{figure}
  \subfigure[Fly 1]{\includegraphics[alt={Bug1}]{example-a}}
  \subfigure[Fly 2]{\includegraphics[alt={Bug2}]{example-b}}
  \caption{caption text}
  \label{fig:one}
  \Description[Fly 1 and Fly 2 look identical]{Fly 1 and fly 2 comparison shows identical length, wingspan, and overall bodily structure.}
  \end{figure}  
\end{document}