Local binarization for TrimmedLasso

[hajicj]

(Related to #118.)

Motivation: make TrimmedLasso tricks work in grayscale images without the need for difficult global binarization.

In order for TrimmedLasso to work, we need to build a mask of the lasso-ed region. We can build this mask dynamically, on some assumptions:

• The foreground we want is inside the lasso region,
• The lasso itself runs mostly through the background.

Easiest solution

Just call otsu binarization on the bounding box and go from there.

[hajicj]

This could be attempted by merging the functionalities of Region Binarize and Trimmed Lasso.