The commit has already been split and commited, this merge is just for metadata.
What has not been merged is the removial of some start/end_frame / draw_slice
related functions that are still in use. They can be removed once they are
unused.

* commit '565e4993':
  lavfi: merge start_frame/draw_slice/end_frame
Merged-by: Michael Niedermayer <michaelni@gmx.at>