Pass options to \columns{} environment in beamer export

[John Hendy <jw.hendy@gmail.com>]

I have an org beamer file with the following header since I generally
prefer top-alignment for frames:

#+latex_class_options: [aspectratio=169,t,presentation,bigger,fleqn]

I have some columns where I want vertically centered text, and am not
sure how to pass the option to \columns{} instead of an individual
\column{}.

Worg says this(http://orgmode.org/worg/exporters/beamer/ox-beamer.html):

"All contiguous environments are automatically wrapped in a columns
environment, although it can be forced at any point by setting the
BEAMER_env property to columns. This might be handy if you want to
pass special options."

But where does that go? I have this:

* Test

** Col1      :BMCOL:
   :PROPERTIES:
   :BEAMER_col: 0.5
   :END:

- blah
- blah
- blah

** Col2      :BMCOL:
   :PROPERTIES:
   :BEAMER_col: 0.5
   :END:

- blah
- blah
- blah

I want this (note the [c] to override my document-wide [t] option):

\begin{frame}[label={sec:orgheadline1}]{Test}
\begin{columns}[c]
\begin{column}{0.5\columnwidth}
\begin{itemize}
\item blah
\item blah
\item blah
\end{itemize}
\end{column}

\begin{column}{0.5\columnwidth}
\begin{itemize}
\item blah
\item blah
\item blah
\end{itemize}
\end{column}
\end{columns}
\end{frame}
\end{document}

But if I add =:beamer_env: columns= to both subheadlines, I get each
in it's own \begin/end{columns} environment, which is not what I want.
I just need a single columns env, with the [c] option passed, that
encompasses both individual column environments.


John