[Groff] Bugs around variable `device'

[Bernd Warken]

The shell command
  echo test | troff -T ps | grotty

produces some random error message such as
  grotty:<standard input>:10:fatal error: width of character not equal
  to horizontal resolution

The right error message, however, should be
  device not suitable for postprocessor

The reason for this flaw is that groff assumes that the device is the
same over the whole command queue.  But that's not true as is shown
above.

If you dig for the error you find some subtle bugs around the global
variable `device' defined in `include/device.h' and used in
`libdriver/input.cc'.  This variable `device' should only be used for
the troff formatting process, but not during the postprocessing.

For the postprocessing, a different variable to store the
postprocessor name should be created, which must be set by each
postprocessor before calling the parser for the intermediate output,
e.g. for grotty
  postprocessor_name = "grotty";

Additionally, a global dictionnary should be provided that maps a
device name to a postprocessor name, e.g.
{
  {"ps", "grops"}
  {"ascii", "grotty"}
  {"latin1", "grotty"}
  \[ell]
}

A natural place to declare these variables is the file `driver.h' that
is included anyway.

This problem exists independently of the output resolution scaling,
but must be fixed first.

Bernd Warken