First shot at old style figures

Very similar to smcp
Remove not-used io.readall implementation

This was supposed to be a faster version of
  f:read("*all")

See http://www.mail-archive.com/dev-luatex@ntg.nl/msg02847.html
New feature "dpiwarn" for low resolution images

If the real dpi is less than the given amount, a waring will be given.