switch to LIVE_STREAM segmentation mode
add deque for holding masks to GreenCompositor
rename GreenCompositor prepare_mask to update_mask
re-organise operations on mask in update_mask
rename GreenCompositor blend member to blend_with_mask
add open, close, play and pause to VirtualSink
add open, close to CameraSource
delete start and stop from VirtualSink and CameraSource
add GPU build instructions