Use SPDX license and copyright identifiers

As mentioned in #82 this MR updates the project to use SPDX identifiers - both license and copyright ones. The HACKING file is also updated, although the file as a whole could use some extra love.

There are a few exceptions/places that have not been updated:

• files imported from elsewhere - cmake module, meson opengl_headers, everything in third_party
• man pages - these are covered under CC BY-SA 3.0 US, although there is no equivalent SPDX identifier (perhaps because there are dozen+ variants 😛 ). Potentially we can re-license the docs under CC-BY-SA 4.0 which is reportedly clearer across jurisdictions - @linyaa what do you think?

Neither of which is a non-blocker.

As an official disclaimer: any copyright or licensing changes in this MR are coincidental accidental. If you spot any, please open an issue or a MR.