pdftoppm (from poppler-utils package) is several orders of
magnitude quicker than ghostscript extracting PNG images
from PDF documents.
We add support for this tool and use it whenever set up,
or using gs as before if missing.
Revisited code with peer review comments. In particular:
1. Make default path for pdftoppm empty string.
2. Fix a typo on the method description for get_gs_command_for_image.
3. Added more information why is useful pdftoppm into string pathtopdftoppm_help.
4. Check that path for pdftoppm is executable to prevent errors. Otherwise, use gs.
96557076a2