The stripes are intended to extend down to the full decoded
height of the frame, which is always a multiple of 8 luma pixels,
in order to avoid some nasty edge cases.

This change was partially implemented in previous patches, but
not everywhere was modified, leading to slightly inconsistent code.
This patch finishes making the relevant changes, along with a
slight bit of refactoring.

Change-Id: Ibc8e2f5ace5415815625edbc224557a7c548c38a