Frame threading

Currently frame threading isn't used anywhere which means that not all code paths are being tested.

closed

mentioned in merge request !80 (merged)

mentioned in merge request !81 (merged)

mentioned in merge request !83 (merged)

mentioned in commit rbultje/dav1d@b571502a

mentioned in commit rbultje/dav1d@1edc9459

mentioned in commit rbultje/dav1d@8cee9463

mentioned in commit rbultje/dav1d@c7de5f45

mentioned in commit rbultje/dav1d@2f813d06

mentioned in commit rbultje/dav1d@15ddd3c9

mentioned in commit rbultje/dav1d@2dea42e0

mentioned in commit rbultje/dav1d@14eec6eb