ext: Checkout libaom v3.1.3

Updates the libaom version checked out in the ext and docker scripts
from v3.1.2 to v3.1.3.

This fixes the build failure on macOS with the Apple M1 chip. See
https://github.com/AOMediaCodec/libavif/pull/759.

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 23070df..59684bb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@
 *   - speed 0-6: no change is needed
 *   - speed 7:   change to speed 6 for the same results
 *   - speed 8-9: re-test and re-adjust speed according to your app needs
+* Update aom.cmd: v3.1.3
 * Update dav1d.cmd: 0.9.2
 
 ## [0.9.2] - 2021-06-23

ext/aom.cmd

diff --git a/ext/aom.cmd b/ext/aom.cmd
index e934d7e..b09516b 100755
--- a/ext/aom.cmd
+++ b/ext/aom.cmd
@@ -8,7 +8,7 @@
 : # If you're running this on Windows, be sure you've already run this (from your VC2019 install dir):
 : #     "C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Auxiliary\Build\vcvars64.bat"
 
-git clone -b v3.1.2 --depth 1 https://aomedia.googlesource.com/aom
+git clone -b v3.1.3 --depth 1 https://aomedia.googlesource.com/aom
 
 cd aom
 mkdir build.libavif

src/codec_aom.c

diff --git a/src/codec_aom.c b/src/codec_aom.c
index 655c168..e68f404 100644
--- a/src/codec_aom.c
+++ b/src/codec_aom.c
@@ -735,7 +735,8 @@
     // do not "align" aomImage.w and aomImage.h. Unfortunately this exposes a bug in libaom
     // (https://crbug.com/aomedia/3113) if chroma is subsampled and image->width or image->height is
     // equal to 1. To work around this libaom bug, we allocate the aomImage.planes[] buffers and
-    // copy the image YUV data if image->width or image->height is equal to 1.
+    // copy the image YUV data if image->width or image->height is equal to 1. This bug has been
+    // fixed in libaom v3.1.3.
     //
     // Note: The exact condition for the bug is
     //   ((image->width == 1) && (chroma is subsampled horizontally)) ||

tests/docker/build.sh

diff --git a/tests/docker/build.sh b/tests/docker/build.sh
index 3674fd7..90239a4 100644
--- a/tests/docker/build.sh
+++ b/tests/docker/build.sh
@@ -34,7 +34,7 @@
 
 # aom
 cd
-git clone -b v3.1.2 --depth 1 https://aomedia.googlesource.com/aom
+git clone -b v3.1.3 --depth 1 https://aomedia.googlesource.com/aom
 cd aom
 mkdir build.avif
 cd build.avif