set mask in Python ?

Tested both **c++** and **python**.

1) In **c++**:
`bgs->process(img_input, img_mask, img_bkgmodel);`
we can input a **img_mask**.

2) In **python**:
from `demo.py` and `demo2.py`.
`img_output = algorithm.apply(frame)`

it looks there is **nowhere** for me to input an **mask image**?

Any suggestions?