Convert "lump" names from directories to lowercase

- Zip and 7z already do this. Directories should too.

src/resourcefiles/file_directory.cpp

@@ -302,8 +302,13 @@ void FDirectory::AddEntry(const char *fullpath, int size)
 
 	// Store the full path here so that we can access the file later, even if it is from a filter directory.
 	lump_p->mFullPath = fullpath;
+
+	// [mxd] Convert name to lowercase
+	FString name = fullpath + strlen(Filename);
+	name.ToLower();
+
 	// The lump's name is only the part relative to the main directory
-	lump_p->LumpNameSetup(fullpath + strlen(Filename));
+	lump_p->LumpNameSetup(name);
 	lump_p->LumpSize = size;
 	lump_p->Owner = this;
 	lump_p->Flags = 0;