This feature got lost with new auto-track API,
Added it back by extending frame accessor class. This isn't really
a frame thing, but we don't have other type of accessor here.
Surely, we can use old-style API here and pass mask via region
tracker options for this particular case, but then it becomes much
less obvious how real auto-tracker will access this mask with old
style API.
So seems we do need an accessor for such data, just matter of
finding better place than frame accessor.
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