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Mostly just worked already, the way that MIPS scaling was implemented
was unnecessarily over-complicated though. Simply always using the Y
plane size when scaling yields the same results.

This also fixes some typos in the AVIF test file names as well as adds
a README.md file for the ones I've updated. Some of which actually
needed to be recreated since they were incorrectly labeled.

Fixed: 915972
Test: Updated unittests.

Change-Id: I7582a41f8dce6af21aaa1b1d2dda182f640297d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318056Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792547}