css: Add a workaround for gtk_widget_override_font()

The problem here is that the CSS machinery expects font sizes to be in
pixels, but gtk_widget_override_font() provides a value in point and the
CSS machinery has no ability to query the DPI and convert.

This patch changes the dconversion DPI we use from a hardcoded 96 to the
default screen's DPI, which should work better than before.
This will of course not listen to changes in the default screen's DPI,
but that shouldn't be a problem.

People who want to workaround this should use gtk_widget_override_font()
with a font that has an absolute size set via
pango_font_description_set_absolute_size (size * PANGO_SCALE *
                                          gdk_screen_get_resolution (screen));

https://bugzilla.gnome.org/show_bug.cgi?id=774248

gtk/gtkcssshorthandpropertyimpl.c

@@ -1118,7 +1118,7 @@ unpack_font_description (GtkCssShorthandProperty *shorthand,
       g_value_init (&v, G_TYPE_DOUBLE);
       size = pango_font_description_get_size (description) / PANGO_SCALE;
       if (!pango_font_description_get_size_is_absolute (description))
-        size = size * 96.0 / 72.0;
+        size = size * gdk_screen_get_resolution (gdk_screen_get_default ()) / 72.0;
       g_value_set_double (&v, size);
       prop = _gtk_style_property_lookup ("font-size");
       _gtk_style_property_assign (prop, props, state, &v);