revealjs: logo placement on title slide vs. every slide

[matthiasbaitsch]

Description

Idea

In revealjs, logo places the logo on every slide. Many institutional designs need it on the title slide instead, or on the title slide in addition to a small one on the other slides.

It would be helpful if the two placements could be controlled separately, e.g.

logo:
  every-slide: false   # current behaviour: logo on every slide
  title-slide: true    # logo on the title slide

(The names are just a suggestion.) Both would use the logo from _brand.yml (logo.small / logo.medium) as today, so a format or extension can set the placement while each project brings its own brand.

Why

• The only built-in way to get a logo onto the title slide is title-slide-attributes: data-background-image: .... It does not use the brand, allows a single image only, and breaks with embed-resources (embed-resources breaks title-slide-attributes in revealjs #7221).
• Hiding the logo on the title slide, or showing it only there, comes up repeatedly: Reveal.js Presentation: Title slide without page number and logo #557, Revealjs: hide logo on the title slide #12820, Use different logo (quarto param) on different slides revealjs #12607, Two images on revealjs title slide #13539. The answers point to custom CSS, Lua filters or JavaScript.
• Extension authors currently write filters for this (embedding the image, resolving paths with output-dir, etc.), although it is mostly a placement switch.

Typst output has the same limitation (the brand logo is always a background on every page), so the same option could apply there later.