Bugfix: image loading fails gracefully

If an image does not exist, we should not ask LuaTeX
to load it anyway. We do this

a) look into the internal hash (as we did before)
   if it's in the hash, everything is fine
b) look if the user supplied a URI scheme (file://)
   - try to find the file on the hard drive. If
   it doesn't exist - return nil (signaling fnf)
   if it exists, return the path.