Fix font size from FontButton, behave differently from Gtk3.18 vs. 3.20

Gtk3.18 use px unit, Gtk3.20 use pt unit. Font selection does not
necessarily select the correct font where variant exists, expect strange
result. Nothing will be done to fix that.

rtgui/main.cc

@@ -338,7 +338,13 @@ int main(int argc, char **argv)
         if (options.fontFamily != "default") {
             try {
                 cssForced = Gtk::CssProvider::create();
+                //GTK318
+                #if GTK_MAJOR_VERSION == 3 && GTK_MINOR_VERSION < 20
+                cssForced->load_from_data (Glib::ustring::compose("* { font-family: %1; font-size: %2px }", options.fontFamily, options.fontSize));
+                #else
                 cssForced->load_from_data (Glib::ustring::compose("* { font-family: %1; font-size: %2pt }", options.fontFamily, options.fontSize));
+                #endif
+                //GTK318
                 Gtk::StyleContext::add_provider_for_screen(screen, cssForced, GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
             } catch (Glib::Error &err) {
                 printf("Error: \"%s\"\n", err.what().c_str());