* Add license text of all licenses used under LICENSES/.
* Add copyright statements that are readable by REUSE where reasonably possible.
* Add .reuse/dep5 entry as fallback licenses for images etc. where explicit copyrights not possible.
* Add 'reuse lint' check to github Actions CI
* Use 'TBD' license on vendor extensions, none of which seem to have explicit licenses and may not be owned by Khronos.
* Some minor whitespace / EOL convention changes.
* Add boilerplate files missing from repo such as CONTRIBUTING and COPYING.