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Per code inspection and commentary from ffmpeg developers, we
need to be using a stride that is aligned to 32 for software
video frames.

It seems at somepoint we might be able to use av_cpu_max_align(),
but even ffmpeg doesn't follow this internally and has exceptions
for when H264 content is used, so just leave a TODO for now.

BUG=773673
TEST=no more ubsan failure.

Change-Id: I012bf7ca5531559df63061fc1edc9fdeaebf4c08
Reviewed-on: https://chromium-review.googlesource.com/801775Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520727}