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.


Review URL:

Cr-Commit-Position: refs/heads/master@{#312420}
2 files changed