overlay issue for vppblending

[feiwan1]

Hi @FurongZhang ,

I developed a patch that can let vppblending use the 1st input surface as the output for 2 inputs overlay case:
feiwan1@5c7c0da

But the output looks incorrect and the 1st input image looks like cleaned even when set output_background_color as 0.
$ vppblending test.cfg (need to rename attach txt to cfg)
test.txt

Output yuv file:

Expected result:

[feiwan1]

CC @xuguangxin @xhaihao @XinfengZhang