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/windows_source_install.md

@@ -43,6 +43,12 @@ python -m pip install OpenImageIO
 python -c "import OpenImageIO"
 ```
 
+To use a PDF file as an image (`@image` of a `.pdf`), Klammertext runs
+poppler's `pdfinfo` and `pdftoppm`; without them such a document stops
+with a message naming the package. MSYS2 (installed below) provides them:
+in the UCRT64 shell `pacman -S mingw-w64-ucrt-x86_64-poppler`, and for use
+from PowerShell put `C:\msys64\ucrt64\bin` on your `Path` as well.
+
 Note that the interpreter is called `python` on Windows. There is no
 `python3`; the `python3` that Windows itself offers is a placeholder that
 opens the Microsoft Store, and Klammertext does not use it.