libaom v3.1.2 Released

[Wan-Teh Chang]

Hello everyone,

libaom v3.1.2 has been released. The source code of the release can be
checked out from the git repository[1] using the release tag v3.1.2.
Alternatively, the release tarball[2], along with the signature file[3],
can be downloaded.

Thanks.

[1] https://aomedia.googlesource.com/aom
[2] https://storage.googleapis.com/aom-releases/libaom-3.1.2.tar.gz
[3] https://storage.googleapis.com/aom-releases/libaom-3.1.2.tar.gz.asc

This release includes several bug fixes.

- Bug fixes:
exports.cmake: use APPLE and WIN32 and use def for mingw-w64
https://aomedia-review.googlesource.com/c/aom/+/139882
NOTE: This bug fix has a follow-up fix, which was NOT cherry-picked:
exports: use CMAKE_SHARED_LIBRARY_PREFIX to determine lib name
https://aomedia-review.googlesource.com/c/aom/+/140661

Issue 2993: Incorrect spatial_id when decoding base layer of
multi-layer stream

Issue 3080: Chroma Resampling by Encoder on Y4M Inputs Files Tagged
as C420mpeg2

Issue 3081: Use of uninitialized value $version_extra in
concatenation (.) or string at aom/build/cmake/version.pl line 88.