Simplify the resource prefix path engine parameter handling.

Also ensure that relative prefix path is being resolved relative to the program dir regardless of where the prefix path parameter value is coming from.

Source/Urho3D/Engine/Engine.cpp

@@ -199,12 +199,11 @@ bool Engine::Initialize(const VariantMap& parameters)
     ResourceCache* cache = GetSubsystem<ResourceCache>();
     FileSystem* fileSystem = GetSubsystem<FileSystem>();
 
-    String defaultPrefixPath(AddTrailingSlash(getenv("URHO3D_PREFIX_PATH")));
-    if (defaultPrefixPath.Empty())
-        defaultPrefixPath = fileSystem->GetProgramDir();
-    else if (!IsAbsolutePath(defaultPrefixPath))
-        defaultPrefixPath = fileSystem->GetProgramDir() + defaultPrefixPath;
-    String resourcePrefixPath = AddTrailingSlash(GetParameter(parameters, "ResourcePrefixPath", defaultPrefixPath).GetString());
+    String resourcePrefixPath = AddTrailingSlash(GetParameter(parameters, "ResourcePrefixPath", getenv("URHO3D_PREFIX_PATH")).GetString());
+    if (resourcePrefixPath.Empty())
+        resourcePrefixPath = fileSystem->GetProgramDir();
+    else if (!IsAbsolutePath(resourcePrefixPath))
+        resourcePrefixPath = fileSystem->GetProgramDir() + resourcePrefixPath;
     Vector<String> resourcePaths = GetParameter(parameters, "ResourcePaths", "Data;CoreData").GetString().Split(';');
     Vector<String> resourcePackages = GetParameter(parameters, "ResourcePackages").GetString().Split(';');
     Vector<String> autoLoadPaths = GetParameter(parameters, "AutoloadPaths", "Autoload").GetString().Split(';');