Merge tag 'v1.13.1'

libvpx 1.13.1

2023-09-29 v1.13.1 "Ugly Duckling"
  This release contains two security related fixes. One each for VP8 and VP9.

  - Upgrading:
    This release is ABI compatible with the previous release.

  - Bug fixes:
    https://crbug.com/1486441 (CVE-2023-5217)
    Fix to a crash related to VP9 encoding (#1642)

* tag 'v1.13.1':
  update CHANGELOG
  update version to 1.13.1
  Fix bug with smaller width bigger size
  vp9_alloccommon: clear allocation sizes on free
  VP8: disallow thread count changes
  encode_api_test: add ConfigResizeChangeThreadCount
  README: update release version to 1.13.0

Bug: webm:1818
Change-Id: I732e2423f635d4115890f00fd63f9886e31f39a6

diff --cc README
Simple merge

diff --cc libs.mk
Simple merge