PDF enhancement: support best-effort format conversions

Bug description

On 1.3.310, with this URL:

https://substackcdn.com/image/fetch/w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%253A%252F%252Fsubstack-post-media.s3.amazonaws.com%252Fpublic%252Fimages%252Fced5d5ad-a0d6-44aa-b6f5-73c0a937d016_3553x5273.jpeg

running xelatex - 1
  This is XeTeX, Version 3.141592653-2.6-0.999995 (TeX Live 2023) (preloaded format=xelatex)
   restricted \write18 enabled.
  entering extended mode
  
updating tlmgr

updating existing packages

compilation failed- error
LaTeX Error: Cannot determine size of graphic in index_files/mediabag/ced5d5ad-a0d6-44aa-b.webp (no BoundingBox).

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.1296 ...iles/mediabag/ced5d5ad-a0d6-44aa-b.webp}

My _quarto.yml:

  pdf:
    papersize: "Letter"
    mainfont: "Georgia"
    documentclass: scrbook
    standalone: true

 quarto list tools
[✓] Inspecting tools

Tool         Status            Installed     Latest
chromium     Not installed     ---           869685
tinytex      Up to date        v2023.04      v2023.04

Checklist

• Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
• Please format your issue so it is easier for us to read the bug report.
• Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
• Please document the operating system you're running. If on Linux, please provide the specific distribution.
• Please provide the output of quarto check so we know which version of quarto and its dependencies you're running.

No source file or test is named. First reproduce the reported xelatex failure with the supplied URL and Quarto PDF settings, then trace the PDF/media conversion entry point; done means the WebP input is handled best-effort and PDF generation no longer fails.