The copy from old buffer is only needed when appending (when
decode_bitstream is called more than once in one frame).
This avoids map old buffer -> memcpy to new buffer which can get
expensive with large buffers.
Reviewed-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29318>
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