thanks to Erik's texture map I can now drop empty.rgb altogether and just

specify the same texture in the "foo.lighted" and "foo.unlighted" material
entry. This also allows to drop the state cloning and thereby solves the
most urgent apt_signs.cxx TODO. :-)

simgear/scene/tgdb/apt_signs.cxx

@@ -224,8 +224,7 @@ ssgBranch *sgMakeSign(SGMaterialLib *matlib, const string path, const string con
 
             SGMaterial *m = matlib->find(u);
             if (m) {
-                unlighted_state = (ssgSimpleState *)m->get_state()->clone(SSG_CLONE_STATE);
-                unlighted_state->setTexture(lighted_state->getTexture());
+                unlighted_state = m->get_state();
             } else {
                 SG_LOG(SG_TERRAIN, SG_ALERT, SIGN "ignoring unknown material `" << u << '\'');
                 unlighted_state = lighted_state;