Merge all composing variables, not just those in destination already

This resolves a regression where the local openmw.cfg doesn't exist, so
we fall back to the global openmw.cfg, but because we've not loaded
anything from the local openmw.cfg, there are no variables with which to
merge.

components/files/configurationmanager.cpp

@@ -87,7 +87,7 @@ boost::program_options::variables_map ConfigurationManager::separateComposingVar
     boost::program_options::variables_map composingVariables;
     for (auto itr = variables.begin(); itr != variables.end();)
     {
-        if (description.find((*itr).first, false).semantic()->is_composing())
+        if (description.find(itr->first, false).semantic()->is_composing())
         {
             composingVariables.emplace(*itr);
             itr = variables.erase(itr);
@@ -100,14 +100,23 @@ boost::program_options::variables_map ConfigurationManager::separateComposingVar
 
 void ConfigurationManager::mergeComposingVariables(boost::program_options::variables_map & first, boost::program_options::variables_map & second, boost::program_options::options_description& description)
 {
-    for (auto& [name, variableValue] : first)
+    for (const auto& option : description.options())
     {
-        if (description.find(name, false).semantic()->is_composing())
+        if (option->semantic()->is_composing())
         {
+            std::string name = option->canonical_display_name();
+
+            auto firstPosition = first.find(name);
+            if (firstPosition == first.end())
+            {
+                first.emplace(name, second[name]);
+                continue;
+            }
+
             if (second[name].defaulted() || second[name].empty())
                 continue;
 
-            boost::any& firstValue = variableValue.value();
+            boost::any& firstValue = firstPosition->second.value();
             const boost::any& secondValue = second[name].value();
             
             if (firstValue.type() == typeid(Files::EscapePathContainer))