libaom v3.1.3 release candidate 2

[Wan-Teh Chang]

Hi everyone,

libaom v3.1.3 release candidate 2 is available. The release tag is
v3.1.3-rc2. Your testing will be greatly appreciated, especially on
macOS with the Apple M1 chip or on Windows with MSVC.

Compared with release candidate 1, we added the fixes for issue 3113 and
issue 3115 and public header (aom/aom_encoder.h) comment updates.

We are planning to create the final release on Monday, Oct 4, 2021.

This release includes several bug fixes.

- Bug fixes:
The following four cmake changes should help the people building
libaom using MSVC.
1. exports: use CMAKE_SHARED_LIBRARY_PREFIX to determine lib name
https://aomedia-review.googlesource.com/c/aom/+/142342
2. aom_install: Install lib dlls to bindir
https://aomedia-review.googlesource.com/c/aom/+/146546
3. aom_install: use relpath for install
https://aomedia-review.googlesource.com/c/aom/+/146550
4. aom_install: don't exclude msvc from install
https://aomedia-review.googlesource.com/c/aom/+/146547

aom/aom_encoder.h: remove configure option reference
https://aomedia-review.googlesource.com/c/aom/+/146743

Issue 3113: Tests for detecting chroma subsampling in
av1_copy_and_extend_frame() do not work when y_width or y_height is
1

Issue 3115: image2yuvconfig() should calculate uv_crop_width and
uv_crop_height from y_crop_width and y_crop_height

Issue 3140: rc_overshoot_pct is documented as having a range of
0-1000, but is range checked against 0-100

Issue 3147: Build failure on Apple M1 arm64

Thanks,
Wan-Teh Chang