enable access to segmentation masks in posedetection

[cdiddy77]

The segmentation mask for each pose is a 2d array of values, an image essentially, which represents the mask. This obviously should not be transferred across the native <=> js boundary on every frame, as it is a 1080p bitmap. But it supplies the information to do basic green-screening of the poser, which certainly has potential.

Somehow the web version does expose it. You can turn it on in the mediapipe web demo.