[NEW] Beamer aspect-ratio, font sizes

[Rob Beezer]

You can now set the aspect ratio for a Beamer slideshow. And in concert with
this we have changed the default from 4:3 to be 16:9. So you may need to adjust
if your slides are designed for 4:3. Note that images will get wider with this
change, and then also taller. So set 4:3 explicitly to preserve the status quo,
or review potential changes.

And you can then set a font size - either to adjust how much content goes on a
slide, or to control how many students sit in the back row.

Start here:

Subsection 45.7.2: Beamer Slide Shape
https://pretextbook.org/doc/guide/html/publication-file-beamer.html#beamer-page-options

Subsection 45.7.3: Beamer Font Sizes
https://pretextbook.org/doc/guide/html/publication-file-beamer.html#beamer-font-size-options