Support saving of image history to support union of masks in tester mode

[himanshunaidu]

In the live capture view, the app does not just process the current image, but every image in the history (generally the last 5 images).
But the dataset encoder does not save these. Thus the tester mode has to make an exception for union of masks at the moment.

[himanshunaidu]

NOTE: We could simulate union of masks for already saved benchmarks, by performing small homography transformations to simulate a history.
One way to do so could be to slightly move the position of the user backwards (camera transform position) while keeping other factors the same, and then performing the required homography transformation for that.