Really unbreak creation of PDF documents which use "shared" PNG images:

The previous fix only worked in some cases, so back it out and add a
more clean fix. This fix also makes "make clean" work as expected again
for the shared PNG images.

The real problem is the assumption in doc.images.mk that the
${IMAGES_PDF} variable only need to contain images converted to PDF
format and not images which are already in PNG format. We need to list
the PNG images in ${IMAGES_PDF} since they might be in a shared image
directory, and if we don't list them in ${IMAGES_PDF} they will not be
copied to the working directory and will therefor not be found during
build.

Prompted by: phantom