Make sure luax_getfile always returns a retained value

Before it would return a retained value if it returned a freshly constructed file, and an unretained value otherwise.
Also update all (3) call sites, to correctly retain or release as needed.

src/modules/video/wrap_Video.cpp

@@ -47,6 +47,7 @@ int w_newVideoStream(lua_State *L)
 
 	luax_pushtype(L, stream);
 	stream->release();
+	file->release();
 	return 1;
 }