Images: a PDF figure finds poppler, the lookup admits image files only, a missing image is an error

Assembled from development commit 8ed24ef03b36. Since ee8bcf5: a PDF file used
as an image finds poppler's pdfinfo and pdftoppm on the PATH instead of
/usr/bin, and says which package to install when they are missing (the
Linux, macOS and Windows pages name it); the image lookup considers
image files only, so a source or log file beside a figure is never taken
for it; and an @image whose file cannot be found is an error naming the
basename and the search path, instead of a silent red placeholder. Two
test suites that had relied on that placeholder now supply a real image;
a new suite covers the PDF figure.

doc/install/linux_source_install.md

@@ -32,6 +32,14 @@ sudo apt-get install python3-openimageio
 python3 -c "import OpenImageIO"
 ```
 
+To use a PDF file as an image (`@image` of a `.pdf`), poppler's `pdfinfo`
+and `pdftoppm` must be on the `PATH`; without them such a document stops
+with a message naming the package:
+
+```bash
+sudo apt-get install poppler-utils
+```
+
 (A `pip` install works too, but a fresh Ubuntu has no `pip` until
 `python3-pip` is installed, and since 24.04 it refuses to install into the
 system Python without `--break-system-packages`; the package avoids both.)