main/aom: upgrade to 3.9.0
https://aomedia.googlesource.com/aom/+/refs/tags/v3.9.0
I ran the tests for both 3.8.2 and 3.9.0. In both cases, the tests fail at AV1/DatarateTestLarge.BasicRateTargetingSuperresCBRMultiThreads/0
. It looks like at least nothing got worse, I guess...
Edited by Hannes Braun