currently, the model outputs the rain, and so need a subtraction
in filter c code to get the final derain result.

I've sent a PR to update the model file and accepted, see at
https://github.com/XueweiMeng/derain_filter/pull/3Signed-off-by: Guo, Yejun <yejun.guo@intel.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>