This formats all copyright comments according to SPDX formatting guidelines.
Additionally, this resolves the remaining GPLv2 only licensed files by relicensing them to GPLv2.0-or-later.
Due to how error prone the container design is, this commit adds unit
tests for it.
Some tests taken from here are based on bugs from using this buffer
container in games, so if we ever break it in the future in a way that
might harm games, the tests should fail.
