PDF export rasterized every slide as one full-page bitmap, so the
background, master and layout graphics a deck repeats were stored once
per slide. Draw each slide as its backdrop plus the slide's own shapes;
equal backdrops become equal images and the writer stores each once.
Keep the keyboard while editing list text: Tab and the arrow keys moved
egui's keyboard focus onto ordinary widgets, and that stray focus
swallowed every later key, so lists could not be typed into or continued
with Enter.
Fabian