Blender Git Loki

Fix T50908: Motion Tracker ignored grease pencil mask

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.