Make background transparent for binary segmentations

[fedorov]

Example: https://healthcare.googleapis.com/v1/projects/idc-sandbox-000/locations/us-central1/datasets/fedorov-dev-healthcare/dicomStores/singlebitsegstest/dicomWeb (limited access).

Currently, it is impossible to change opacity of the background without changing the opacity of the foreground. We need to have same functionality as while showing segmentations in OHIF.