handle unknown metadata OBU type not correctly
In our code we just drop the whole data: https://code.videolan.org/videolan/dav1d/-/blob/master/src/obu.c#L1533.
But libgav1 which google uses handles it as av1 spec describes: https://chromium.googlesource.com/codecs/libgav1/+/refs/heads/main/src/obu_parser.cc#2472.
Sample video bilibili-av1-test.mp4 can be found at https://github.com/rockchip-linux/mpp/issues/351.
Edited by Jianfeng Liu