fix sparse shape threshold

[evaleev]

handling of thresholds by SparseShape was inconsistent. For example:

• SparseShape::mask would produce a shape screened using this->my_threshold_, whereas other unary operations would use global threshold
• SparseShape::update_block would screen the result using global threshold and use this->my_threshold_ to construct the result

Now all unary operations use this->init_threshold() (returning this->my_threshold_) for the result and all binary operations use global threshold.