Currently it always copies the metadata fields from the last input when
there are multiple inputs for the filter. For example, the metadata
fields from input1 are copied to the output for overlay_qsv filter,
however for regular overlay filters, the metadata fields from input0 are
copied to the output. With this fix, we may copy the metadata fields
from input0 to the ouput as well.
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>