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.
It is disabled by default, so should not affect existing configurations.
Main benefits of this goes as:
- Linux distros can use that to avoid libraries duplication and link
blender package against gflags package from the system.
- It it easier to test whether Blender works with updated version of
Gflags prior to re-bundling the library.
