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/macos_source_install.md

@@ -58,6 +58,11 @@ With pip, the self-contained PyPI wheel goes into that Python's site-packages;
 managed". (Homebrew alternative: `brew install openimageio`, heavier — it pulls
 ffmpeg/openexr/etc.)
 
+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: `brew install poppler` (MacPorts: `sudo port
+install poppler`).
+
 ## 3. Clone and configure the environment
 
 ```sh