contrib: aom: update to 3.1.2
- Bug fixes [1]:
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exports.cmake: use APPLE and WIN32 and use def for mingw-w64 https://aomedia-review.googlesource.com/c/aom/+/139882
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Issue 2993: Incorrect spatial_id when decoding base layer of multi-layer stream
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Issue 3080: Chroma Resampling by Encoder on Y4M Inputs Files Tagged as C420mpeg2
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Issue 3081: Use of uninitialized value $version_extra in concatenation (.) or string at aom/build/cmake/version.pl line 88.
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[1] https://aomedia.googlesource.com/aom/+/refs/tags/v3.1.2/CHANGELOG