Overview:
If the unit of length is chosen as millimeters in the gtkmm class
Gtk::PrintOperation, pango_layout_line_get_extents reports a too large logical
height of empty lines. (The unit of length is transferred to pango in a call
to pango_cairo_context_set_resolution.)

   Steps to reproduce:
Start with the simple printing example in the tutorial "Programming with
gtkmm" at http://git.gnome.org/browse/gtkmm-documentation/tree/examples/book/printing/simple.

In file examplewindow.cc, function ExampleWindow::print_or_preview, after
  print->set_print_settings(m_refSettings);
add
  print->set_unit(Gtk::UNIT_MM);

Run the program, enter some text in the text box 'Comments'. Enter an empty
line between non-empty lines. For easy comparison, you may also enter a line
with just a space character. Its height will be correct.
Choose File|Print Preview.

   Actual results:
Empty lines are much higher than non-empty lines. 

   Expected results:
Empty lines should have the same height as non-empty lines.

   Additional information:
With UNIT_PIXEL and UNIT_POINTS all line heights are correct. With UNIT_INCH
it gets even worse than with UNIT_MM. (Gtk::PrintOperation assumes that
dpi = 72, which means that there is actually no difference between pixels and
points.)

pango_layout_line_get_extents calls pango_layout_run_get_extents for non-empty
lines, but pango_layout_line_get_empty_extents for empty lines.
Perhaps pango_layout_line_get_empty_extents does not consider the Cairo
context resolution?