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.
There was some stupidness in the way how tracks are synchronized from the job
to actual DNA data leading to all sort of weird and wonderful failures again.
Writing to the tracks was already inside the lock section, but
reading was not. This could have lead to race condition leading
to all sorts of weird and wonderful artifacts.
This was never ported to a new tracking pipeline and now it's done using
FrameAccessor::Transform routines. Quite striaghtforward, but i've changed
order of grayscale conversion in blender side with call of transform callback.
This way it's much easier to perform rescaling in libmv side.
The title actually tells it all, this commit switches Blender to use the new
autotrack API from Libmv.
From the user point of view it means that prediction model is now used when
tracking which gives really nice results.
All the other changes are not really visible for users, those are just frame
accessors, caches and so for the new API.
