Update copyright headers for 2021

[barrbrain]

This builds on #2749 and includes header updates for individual files.

Files requiring updates were identified with the following one-liner:

git grep -L '2021, The rav1e contributors' \
$(git grep -l ', The rav1e contributors' -- \
$(git log --since=2021-01-01 --name-only --pretty= | sort -u))

[coveralls]

Coverage decreased (-0.3%) to 83.698% when pulling 7960160 on barrbrain:update-copyright-headers into 984515f on xiph:master.