libaom v3.12.1 release candidate 1

[Wan-Teh Chang]

Hi everyone,

libaom v3.12.1 release candidate 1 is available. The release tag is
v3.12.1-rc1. Your testing will be greatly appreciated.

This release includes several bug fixes. This release is ABI
compatible with the last release. See
https://aomedia.googlesource.com/aom/+log/v3.12.0..v3.12.1-rc1 for all the
commits in this release.

- Bug Fixes
* b:396169342: Assertion
`av1_is_subpelmv_in_range(&ms_params.mv_limits, start_mv)' failed.
* b:401671154: typo in void init_src_params(...)
* Coverity defect 323670: Uninitialized scalar variable in
encode_with_and_without_superres()
* cmake: bump minimum version to 3.16
* cfl_ppc: fix subtract_average_vsx
* Fix an incorrect index in av1_highbd_pixel_proj_error_neon

Thanks,
Wan-Teh Chang