Reject vp8 video having alpha and image planes of different sizes.

Reject vp8 video having alpha and image planes of different sizes.

Previously we would accept malformed vp8 video files that had alpha and
image planes with different dimensions. Now they result in a decode error.
Also use the alpha image stride when copying the alpha plane, because it
technically doesn't have to be the same as the image stride.

BUG=449958
TEST=ffmpeg_regression_tests

Committed: https://crrev.com/c1a91a8a6a7132c47a174054f0fb56cc3dc8c069
Cr-Commit-Position: refs/heads/master@{#312420}

[watk, 2015-01-21 18:47:48]

watk@chromium.org changed reviewers:
+ dalecurtis@chromium.org, vigneshv@chromium.org

[vignesh, 2015-01-21 18:50:16]

LGTM. thanks watk@.

Although, you will need Dale's LGTM for media owners.

[DaleCurtis, 2015-01-21 19:10:46]

lgtm

[watk, 2015-01-21 19:59:06]

The CQ bit was checked by watk@chromium.org

[commit-bot: I haz the power, 2015-01-21 20:01:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/858303002/1

[commit-bot: I haz the power, 2015-01-21 20:25:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-21 20:27:44]

Patchset 1 (id:??) landed as
https://crrev.com/c1a91a8a6a7132c47a174054f0fb56cc3dc8c069
Cr-Commit-Position: refs/heads/master@{#312420}