Effects: pass model XML file path, not dir path

Pass the full path to the XML file, not is containg directory, through
effects, so that it's available for error reporting. Convert to an
actual directory when using the path to lookup other resources.

simgear/scene/material/makeEffect.cxx

@@ -116,7 +116,7 @@ Effect* makeEffect(const string& name,
     }
     string effectFileName(name);
     effectFileName += ".eff";
-    string absFileName = SGModelLib::findDataFile(effectFileName, options, modelPath);
+    string absFileName = SGModelLib::findDataFile(effectFileName, options, modelPath.dir());
     if (absFileName.empty()) {
         simgear::reportFailure(simgear::LoadFailure::NotFound, simgear::ErrorCode::LoadEffectsShaders, "Couldn't find Effect:" + effectFileName);
         return nullptr;

simgear/scene/model/SGReaderWriterXML.cxx

@@ -801,7 +801,7 @@ sgLoad3DModel_internal(const SGPath& path,
     makeEffectAnimations(animation_nodes, effect_nodes);
     {
         ref_ptr<Node> modelWithEffects = instantiateEffects(group.get(), effect_nodes, options.get(),
-                                                            path.dirPath());
+                                                            path);
         group = static_cast<Group*>(modelWithEffects.get());
     }