Fix 1779616: prevent image span, resolution, etc. from quite reaching zero.
drwxr-xr-x - ETL
drwxr-xr-x - autobuild
drwxr-xr-x - gtkmm-osx
drwxr-xr-x - synfig-core
drwxr-xr-x - synfig-docs
drwxr-xr-x - synfig-osx
drwxr-xr-x - synfig-studio