did some cleanup of the FResourceFile interface.

* making all members protected (but adding friend overrides for the classes which still need it)
* allowing to read data without retrieving the FResourceLump object.

src/common/filesystem/source/filesystem.cpp

@@ -1421,7 +1421,7 @@ FileReader *FileSystem::GetFileReader(int rfnum)
 		return NULL;
 	}
 
-	return Files[rfnum]->GetReader();
+	return Files[rfnum]->GetContainerReader();
 }
 
 //==========================================================================