Theme logo image is rendered without dimensions

I wonder if this was an unexpected regression from #908282: Remove unnecessary I/O from theme_image() → (although that issue was good in general).

The main issue with this is that we don't have anywhere obvious to store the dimensions, the theme settings just point to a file on disk, there's no file reference or similar.

We could do what #908282: Remove unnecessary I/O from theme_image() → used to do in preprocess and cache it somewhere maybe? It's a bit hacky, but it's minimal and would help page rendering quite a lot.

Or we could add dimensions to the theme configuration form/config object and prepopulate them from the file info if not explicitly set? This would have the advantage of not needing an extra cache get but you could end up with weird issues if you change your theme logo file without updating the form.

Could we render the logo alone in its own template, preprocessing the height/width, and leverage the render cache?

I couldn't remember where the logo is rendered, but it's in SystemBrandingBlock - we could do it directly in there I think, just add the attributes when building the render array. It might even be OK to rely on render caching for of the block, although would need to check that.

Made a start on this but it's quite painful.

The pain is mostly because of system_preprocess_block and

core/modules/system/templates/block--system-branding-block.html.twig

In SystemBrandingBlock we do things 'properly' and use '#theme' => 'image', but then that preprocess and the template just extracts the URI from the image render array and renders the image with hard-coded HTML. Why?

This means any added attributes get stripped.

Promoting to a bug because there's no way at all to do this generically at the moment.

I got around this in stark by adding an extra variable for the rendered image, then using that in the template, it is really ugly but I can't think of any other way except adding an entirely new block and deprecating the old one.

Deprecated the 'site_logo' variable, adding a change record and a release note, this ought to be reviewable now.

That svg logic isn't happy, removed that and also switched to using the image factory for non-svg images.

Trying to remove the 'See more' from the snippet