multimedia/libvpx: update 1.8.0
www/firefox, www/firefox-esr, mail/thunderbird wait for upstream fix via
https://bugzilla.mozilla.org/show_bug.cgi?id=1525393
Changes: https://chromium.googlesource.com/webm/libvpx/+log/v1.7.0..v1.8.0
ABI: https://abi-laboratory.pro/tracker/timeline/libvpx/