webp: don't reject VP8X that isn't just VP8 + alpha

[narrowizard]

We already support VP8 + alpha, but reject e.g. VP8 + EXIF. After this
commit, we still don't implement VP8 + EXIF (or ANIM, ICCP, etc.), but
we now silently ignore the EXIF chunk instead of rejecting it.

Fixes golang/go#25738, golang/go#38341

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Patch Set 1:

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Patch Set 11:

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Patch Set 11: Trust+1

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This PR is being closed because golang.org/cl/249445 has been merged.